// Copyright (c) 2014, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

import 'dart:collection';
import 'dart:math' as math;

import 'package:analyzer/dart/ast/ast.dart';
import 'package:analyzer/dart/ast/syntactic_entity.dart';
import 'package:analyzer/dart/ast/token.dart';
import 'package:analyzer/dart/element/element.dart';
import 'package:analyzer/dart/element/type.dart';
import 'package:analyzer/error/error.dart';
import 'package:analyzer/error/listener.dart';
import 'package:analyzer/exception/exception.dart';
import 'package:analyzer/src/dart/ast/token.dart';
import 'package:analyzer/src/dart/ast/utilities.dart';
import 'package:analyzer/src/dart/constant/constant_verifier.dart';
import 'package:analyzer/src/dart/element/element.dart';
import 'package:analyzer/src/dart/element/type.dart';
import 'package:analyzer/src/error/codes.dart';
import 'package:analyzer/src/fasta/token_utils.dart' as util show findPrevious;
import 'package:analyzer/src/generated/engine.dart' show AnalysisEngine;
import 'package:analyzer/src/generated/engine.dart';
import 'package:analyzer/src/generated/java_engine.dart';
import 'package:analyzer/src/generated/parser.dart';
import 'package:analyzer/src/generated/source.dart' show LineInfo, Source;
import 'package:analyzer/src/generated/utilities_dart.dart';

/**
 * Two or more string literals that are implicitly concatenated because of being
 * adjacent (separated only by whitespace).
 *
 * While the grammar only allows adjacent strings when all of the strings are of
 * the same kind (single line or multi-line), this class doesn't enforce that
 * restriction.
 *
 *    adjacentStrings ::=
 *        [StringLiteral] [StringLiteral]+
 */
class AdjacentStringsImpl extends StringLiteralImpl implements AdjacentStrings {
  /**
   * The strings that are implicitly concatenated.
   */
  NodeList<StringLiteral> _strings;

  /**
   * Initialize a newly created list of adjacent strings. To be syntactically
   * valid, the list of [strings] must contain at least two elements.
   */
  AdjacentStringsImpl(List<StringLiteral> strings) {
    _strings = new NodeListImpl<StringLiteral>(this, strings);
  }

  @override
  Token get beginToken => _strings.beginToken;

  @override
  Iterable<SyntacticEntity> get childEntities =>
      new ChildEntities()..addAll(_strings);

  @override
  Token get endToken => _strings.endToken;

  @override
  NodeList<StringLiteral> get strings => _strings;

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitAdjacentStrings(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _strings.accept(visitor);
  }

  @override
  void _appendStringValue(StringBuffer buffer) {
    int length = strings.length;
    for (int i = 0; i < length; i++) {
      StringLiteralImpl stringLiteral = strings[i];
      stringLiteral._appendStringValue(buffer);
    }
  }
}

/**
 * An AST node that can be annotated with both a documentation comment and a
 * list of annotations.
 */
abstract class AnnotatedNodeImpl extends AstNodeImpl implements AnnotatedNode {
  /**
   * The documentation comment associated with this node, or `null` if this node
   * does not have a documentation comment associated with it.
   */
  CommentImpl _comment;

  /**
   * The annotations associated with this node.
   */
  NodeList<Annotation> _metadata;

  /**
   * Initialize a newly created annotated node. Either or both of the [comment]
   * and [metadata] can be `null` if the node does not have the corresponding
   * attribute.
   */
  AnnotatedNodeImpl(CommentImpl comment, List<Annotation> metadata) {
    _comment = _becomeParentOf(comment);
    _metadata = new NodeListImpl<Annotation>(this, metadata);
  }

  @override
  Token get beginToken {
    if (_comment == null) {
      if (_metadata.isEmpty) {
        return firstTokenAfterCommentAndMetadata;
      }
      return _metadata.beginToken;
    } else if (_metadata.isEmpty) {
      return _comment.beginToken;
    }
    Token commentToken = _comment.beginToken;
    Token metadataToken = _metadata.beginToken;
    if (commentToken.offset < metadataToken.offset) {
      return commentToken;
    }
    return metadataToken;
  }

  @override
  Comment get documentationComment => _comment;

  @override
  void set documentationComment(Comment comment) {
    _comment = _becomeParentOf(comment as CommentImpl);
  }

  @override
  NodeList<Annotation> get metadata => _metadata;

  @override
  List<AstNode> get sortedCommentAndAnnotations {
    return <AstNode>[]
      ..add(_comment)
      ..addAll(_metadata)
      ..sort(AstNode.LEXICAL_ORDER);
  }

  /**
   * Return a holder of child entities that subclasses can add to.
   */
  ChildEntities get _childEntities {
    ChildEntities result = new ChildEntities();
    if (_commentIsBeforeAnnotations()) {
      result
        ..add(_comment)
        ..addAll(_metadata);
    } else {
      result.addAll(sortedCommentAndAnnotations);
    }
    return result;
  }

  @override
  void visitChildren(AstVisitor visitor) {
    if (_commentIsBeforeAnnotations()) {
      _comment?.accept(visitor);
      _metadata.accept(visitor);
    } else {
      List<AstNode> children = sortedCommentAndAnnotations;
      int length = children.length;
      for (int i = 0; i < length; i++) {
        children[i].accept(visitor);
      }
    }
  }

  /**
   * Return `true` if there are no annotations before the comment. Note that a
   * result of `true` does not imply that there is a comment, nor that there are
   * annotations associated with this node.
   */
  bool _commentIsBeforeAnnotations() {
    if (_comment == null || _metadata.isEmpty) {
      return true;
    }
    Annotation firstAnnotation = _metadata[0];
    return _comment.offset < firstAnnotation.offset;
  }
}

/**
 * An annotation that can be associated with an AST node.
 *
 *    metadata ::=
 *        annotation*
 *
 *    annotation ::=
 *        '@' [Identifier] ('.' [SimpleIdentifier])? [ArgumentList]?
 */
class AnnotationImpl extends AstNodeImpl implements Annotation {
  /**
   * The at sign that introduced the annotation.
   */
  @override
  Token atSign;

  /**
   * The name of the class defining the constructor that is being invoked or the
   * name of the field that is being referenced.
   */
  IdentifierImpl _name;

  /**
   * The period before the constructor name, or `null` if this annotation is not
   * the invocation of a named constructor.
   */
  @override
  Token period;

  /**
   * The name of the constructor being invoked, or `null` if this annotation is
   * not the invocation of a named constructor.
   */
  SimpleIdentifierImpl _constructorName;

  /**
   * The arguments to the constructor being invoked, or `null` if this
   * annotation is not the invocation of a constructor.
   */
  ArgumentListImpl _arguments;

  /**
   * The element associated with this annotation, or `null` if the AST structure
   * has not been resolved or if this annotation could not be resolved.
   */
  Element _element;

  /**
   * The element annotation representing this annotation in the element model.
   */
  @override
  ElementAnnotation elementAnnotation;

  /**
   * Initialize a newly created annotation. Both the [period] and the
   * [constructorName] can be `null` if the annotation is not referencing a
   * named constructor. The [arguments] can be `null` if the annotation is not
   * referencing a constructor.
   */
  AnnotationImpl(this.atSign, IdentifierImpl name, this.period,
      SimpleIdentifierImpl constructorName, ArgumentListImpl arguments) {
    _name = _becomeParentOf(name);
    _constructorName = _becomeParentOf(constructorName);
    _arguments = _becomeParentOf(arguments);
  }

  @override
  ArgumentList get arguments => _arguments;

  @override
  void set arguments(ArgumentList arguments) {
    _arguments = _becomeParentOf(arguments as ArgumentListImpl);
  }

  @override
  Token get beginToken => atSign;

  @override
  Iterable<SyntacticEntity> get childEntities => new ChildEntities()
    ..add(atSign)
    ..add(_name)
    ..add(period)
    ..add(_constructorName)
    ..add(_arguments);

  @override
  SimpleIdentifier get constructorName => _constructorName;

  @override
  void set constructorName(SimpleIdentifier name) {
    _constructorName = _becomeParentOf(name as SimpleIdentifierImpl);
  }

  @override
  Element get element {
    if (_element != null) {
      return _element;
    } else if (_constructorName == null && _name != null) {
      return _name.staticElement;
    }
    return null;
  }

  @override
  void set element(Element element) {
    _element = element;
  }

  @override
  Token get endToken {
    if (_arguments != null) {
      return _arguments.endToken;
    } else if (_constructorName != null) {
      return _constructorName.endToken;
    }
    return _name.endToken;
  }

  @override
  Identifier get name => _name;

  @override
  void set name(Identifier name) {
    _name = _becomeParentOf(name as IdentifierImpl);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitAnnotation(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _name?.accept(visitor);
    _constructorName?.accept(visitor);
    _arguments?.accept(visitor);
  }
}

/**
 * A list of arguments in the invocation of an executable element (that is, a
 * function, method, or constructor).
 *
 *    argumentList ::=
 *        '(' arguments? ')'
 *
 *    arguments ::=
 *        [NamedExpression] (',' [NamedExpression])*
 *      | [Expression] (',' [Expression])* (',' [NamedExpression])*
 */
class ArgumentListImpl extends AstNodeImpl implements ArgumentList {
  /**
   * The left parenthesis.
   */
  @override
  Token leftParenthesis;

  /**
   * The expressions producing the values of the arguments.
   */
  NodeList<Expression> _arguments;

  /**
   * The right parenthesis.
   */
  @override
  Token rightParenthesis;

  /**
   * A list containing the elements representing the parameters corresponding to
   * each of the arguments in this list, or `null` if the AST has not been
   * resolved or if the function or method being invoked could not be determined
   * based on static type information. The list must be the same length as the
   * number of arguments, but can contain `null` entries if a given argument
   * does not correspond to a formal parameter.
   */
  List<ParameterElement> _correspondingStaticParameters;

  /**
   * Initialize a newly created list of arguments. The list of [arguments] can
   * be `null` if there are no arguments.
   */
  ArgumentListImpl(
      this.leftParenthesis, List<Expression> arguments, this.rightParenthesis) {
    _arguments = new NodeListImpl<Expression>(this, arguments);
  }

  @override
  NodeList<Expression> get arguments => _arguments;

  @override
  Token get beginToken => leftParenthesis;

  @override
  // TODO(paulberry): Add commas.
  Iterable<SyntacticEntity> get childEntities => new ChildEntities()
    ..add(leftParenthesis)
    ..addAll(_arguments)
    ..add(rightParenthesis);

  @deprecated
  List<ParameterElement> get correspondingPropagatedParameters => null;

  @deprecated
  @override
  void set correspondingPropagatedParameters(
      List<ParameterElement> parameters) {
    if (parameters != null && parameters.length != _arguments.length) {
      throw new ArgumentError(
          "Expected ${_arguments.length} parameters, not ${parameters.length}");
    }
  }

  List<ParameterElement> get correspondingStaticParameters =>
      _correspondingStaticParameters;

  @override
  void set correspondingStaticParameters(List<ParameterElement> parameters) {
    if (parameters != null && parameters.length != _arguments.length) {
      throw new ArgumentError(
          "Expected ${_arguments.length} parameters, not ${parameters.length}");
    }
    _correspondingStaticParameters = parameters;
  }

  @override
  Token get endToken => rightParenthesis;

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitArgumentList(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _arguments.accept(visitor);
  }

  /**
   * If
   * * the given [expression] is a child of this list,
   * * the AST structure has been resolved,
   * * the function being invoked is known based on static type information, and
   * * the expression corresponds to one of the parameters of the function being
   *   invoked,
   * then return the parameter element representing the parameter to which the
   * value of the given expression will be bound. Otherwise, return `null`.
   */
  ParameterElement _getStaticParameterElementFor(Expression expression) {
    if (_correspondingStaticParameters == null ||
        _correspondingStaticParameters.length != _arguments.length) {
      // Either the AST structure has not been resolved, the invocation of which
      // this list is a part could not be resolved, or the argument list was
      // modified after the parameters were set.
      return null;
    }
    int index = _arguments.indexOf(expression);
    if (index < 0) {
      // The expression isn't a child of this node.
      return null;
    }
    return _correspondingStaticParameters[index];
  }
}

/**
 * An as expression.
 *
 *    asExpression ::=
 *        [Expression] 'as' [TypeName]
 */
class AsExpressionImpl extends ExpressionImpl implements AsExpression {
  /**
   * The expression used to compute the value being cast.
   */
  ExpressionImpl _expression;

  /**
   * The 'as' operator.
   */
  @override
  Token asOperator;

  /**
   * The type being cast to.
   */
  TypeAnnotationImpl _type;

  /**
   * Initialize a newly created as expression.
   */
  AsExpressionImpl(
      ExpressionImpl expression, this.asOperator, TypeAnnotationImpl type) {
    _expression = _becomeParentOf(expression);
    _type = _becomeParentOf(type);
  }

  @override
  Token get beginToken => _expression.beginToken;

  @override
  Iterable<SyntacticEntity> get childEntities =>
      new ChildEntities()..add(_expression)..add(asOperator)..add(_type);

  @override
  Token get endToken => _type.endToken;

  @override
  Expression get expression => _expression;

  @override
  void set expression(Expression expression) {
    _expression = _becomeParentOf(expression as ExpressionImpl);
  }

  @override
  int get precedence => 7;

  @override
  TypeAnnotation get type => _type;

  @override
  void set type(TypeAnnotation type) {
    _type = _becomeParentOf(type as TypeAnnotationImpl);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitAsExpression(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _expression?.accept(visitor);
    _type?.accept(visitor);
  }
}

/**
 * An assert in the initializer list of a constructor.
 *
 *    assertInitializer ::=
 *        'assert' '(' [Expression] (',' [Expression])? ')'
 */
class AssertInitializerImpl extends ConstructorInitializerImpl
    implements AssertInitializer {
  @override
  Token assertKeyword;

  @override
  Token leftParenthesis;

  /**
   * The condition that is being asserted to be `true`.
   */
  ExpressionImpl _condition;

  @override
  Token comma;

  /**
   * The message to report if the assertion fails, or `null` if no message was
   * supplied.
   */
  ExpressionImpl _message;

  @override
  Token rightParenthesis;

  /**
   * Initialize a newly created assert initializer.
   */
  AssertInitializerImpl(
      this.assertKeyword,
      this.leftParenthesis,
      ExpressionImpl condition,
      this.comma,
      ExpressionImpl message,
      this.rightParenthesis) {
    _condition = _becomeParentOf(condition);
    _message = _becomeParentOf(message);
  }

  @override
  Token get beginToken => assertKeyword;

  @override
  Iterable<SyntacticEntity> get childEntities => new ChildEntities()
    ..add(assertKeyword)
    ..add(leftParenthesis)
    ..add(_condition)
    ..add(comma)
    ..add(_message)
    ..add(rightParenthesis);

  @override
  Expression get condition => _condition;

  @override
  void set condition(Expression condition) {
    _condition = _becomeParentOf(condition as ExpressionImpl);
  }

  @override
  Token get endToken => rightParenthesis;

  @override
  Expression get message => _message;

  @override
  void set message(Expression expression) {
    _message = _becomeParentOf(expression as ExpressionImpl);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitAssertInitializer(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _condition?.accept(visitor);
    message?.accept(visitor);
  }
}

/**
 * An assert statement.
 *
 *    assertStatement ::=
 *        'assert' '(' [Expression] ')' ';'
 */
class AssertStatementImpl extends StatementImpl implements AssertStatement {
  @override
  Token assertKeyword;

  @override
  Token leftParenthesis;

  /**
   * The condition that is being asserted to be `true`.
   */
  ExpressionImpl _condition;

  @override
  Token comma;

  /**
   * The message to report if the assertion fails, or `null` if no message was
   * supplied.
   */
  ExpressionImpl _message;

  @override
  Token rightParenthesis;

  @override
  Token semicolon;

  /**
   * Initialize a newly created assert statement.
   */
  AssertStatementImpl(
      this.assertKeyword,
      this.leftParenthesis,
      ExpressionImpl condition,
      this.comma,
      ExpressionImpl message,
      this.rightParenthesis,
      this.semicolon) {
    _condition = _becomeParentOf(condition);
    _message = _becomeParentOf(message);
  }

  @override
  Token get beginToken => assertKeyword;

  @override
  Iterable<SyntacticEntity> get childEntities => new ChildEntities()
    ..add(assertKeyword)
    ..add(leftParenthesis)
    ..add(_condition)
    ..add(comma)
    ..add(_message)
    ..add(rightParenthesis)
    ..add(semicolon);

  @override
  Expression get condition => _condition;

  @override
  void set condition(Expression condition) {
    _condition = _becomeParentOf(condition as ExpressionImpl);
  }

  @override
  Token get endToken => semicolon;

  @override
  Expression get message => _message;

  @override
  void set message(Expression expression) {
    _message = _becomeParentOf(expression as ExpressionImpl);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitAssertStatement(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _condition?.accept(visitor);
    message?.accept(visitor);
  }
}

/**
 * An assignment expression.
 *
 *    assignmentExpression ::=
 *        [Expression] operator [Expression]
 */
class AssignmentExpressionImpl extends ExpressionImpl
    implements AssignmentExpression {
  /**
   * The expression used to compute the left hand side.
   */
  ExpressionImpl _leftHandSide;

  /**
   * The assignment operator being applied.
   */
  @override
  Token operator;

  /**
   * The expression used to compute the right hand side.
   */
  ExpressionImpl _rightHandSide;

  /**
   * The element associated with the operator based on the static type of the
   * left-hand-side, or `null` if the AST structure has not been resolved, if
   * the operator is not a compound operator, or if the operator could not be
   * resolved.
   */
  @override
  MethodElement staticElement;

  /**
   * Initialize a newly created assignment expression.
   */
  AssignmentExpressionImpl(ExpressionImpl leftHandSide, this.operator,
      ExpressionImpl rightHandSide) {
    if (leftHandSide == null || rightHandSide == null) {
      String message;
      if (leftHandSide == null) {
        if (rightHandSide == null) {
          message = "Both the left-hand and right-hand sides are null";
        } else {
          message = "The left-hand size is null";
        }
      } else {
        message = "The right-hand size is null";
      }
      AnalysisEngine.instance.logger.logError(
          message, new CaughtException(new AnalysisException(message), null));
    }
    _leftHandSide = _becomeParentOf(leftHandSide);
    _rightHandSide = _becomeParentOf(rightHandSide);
  }

  @override
  Token get beginToken => _leftHandSide.beginToken;

  @override
  @deprecated
  MethodElement get bestElement => staticElement;

  @override
  Iterable<SyntacticEntity> get childEntities => new ChildEntities()
    ..add(_leftHandSide)
    ..add(operator)
    ..add(_rightHandSide);

  @override
  Token get endToken => _rightHandSide.endToken;

  @override
  Expression get leftHandSide => _leftHandSide;

  @override
  void set leftHandSide(Expression expression) {
    _leftHandSide = _becomeParentOf(expression as ExpressionImpl);
  }

  @override
  int get precedence => 1;

  @deprecated
  @override
  MethodElement get propagatedElement => null;

  @deprecated
  @override
  set propagatedElement(MethodElement element) {}

  @override
  Expression get rightHandSide => _rightHandSide;

  @override
  void set rightHandSide(Expression expression) {
    _rightHandSide = _becomeParentOf(expression as ExpressionImpl);
  }

  /**
   * If the AST structure has been resolved, and the function being invoked is
   * known based on static type information, then return the parameter element
   * representing the parameter to which the value of the right operand will be
   * bound. Otherwise, return `null`.
   */
  ParameterElement get _staticParameterElementForRightHandSide {
    ExecutableElement executableElement = null;
    if (staticElement != null) {
      executableElement = staticElement;
    } else {
      Expression left = _leftHandSide;
      if (left is Identifier) {
        Element leftElement = left.staticElement;
        if (leftElement is ExecutableElement) {
          executableElement = leftElement;
        }
      } else if (left is PropertyAccess) {
        Element leftElement = left.propertyName.staticElement;
        if (leftElement is ExecutableElement) {
          executableElement = leftElement;
        }
      }
    }
    if (executableElement == null) {
      return null;
    }
    List<ParameterElement> parameters = executableElement.parameters;
    if (parameters.length < 1) {
      return null;
    }
    return parameters[0];
  }

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitAssignmentExpression(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _leftHandSide?.accept(visitor);
    _rightHandSide?.accept(visitor);
  }
}

/**
 * A node in the AST structure for a Dart program.
 */
abstract class AstNodeImpl implements AstNode {
  /**
   * The parent of the node, or `null` if the node is the root of an AST
   * structure.
   */
  AstNode _parent;

  /**
   * A table mapping the names of properties to their values, or `null` if this
   * node does not have any properties associated with it.
   */
  Map<String, Object> _propertyMap;

  @override
  int get end => offset + length;

  @override
  bool get isSynthetic => false;

  @override
  int get length {
    Token beginToken = this.beginToken;
    Token endToken = this.endToken;
    if (beginToken == null || endToken == null) {
      return -1;
    }
    return endToken.offset + endToken.length - beginToken.offset;
  }

  @override
  int get offset {
    Token beginToken = this.beginToken;
    if (beginToken == null) {
      return -1;
    }
    return beginToken.offset;
  }

  @override
  AstNode get parent => _parent;

  @override
  AstNode get root {
    AstNode root = this;
    AstNode parent = this.parent;
    while (parent != null) {
      root = parent;
      parent = root.parent;
    }
    return root;
  }

  Token findPrevious(Token target) =>
      util.findPrevious(beginToken, target) ?? parent?.findPrevious(target);

  @override
  @deprecated
  E getAncestor<E extends AstNode>(Predicate<AstNode> predicate) =>
      thisOrAncestorMatching(predicate);

  @override
  E getProperty<E>(String name) {
    if (_propertyMap == null) {
      return null;
    }
    return _propertyMap[name] as E;
  }

  @override
  void setProperty(String name, Object value) {
    if (value == null) {
      if (_propertyMap != null) {
        _propertyMap.remove(name);
        if (_propertyMap.isEmpty) {
          _propertyMap = null;
        }
      }
    } else {
      if (_propertyMap == null) {
        _propertyMap = new HashMap<String, Object>();
      }
      _propertyMap[name] = value;
    }
  }

  @override
  E thisOrAncestorMatching<E extends AstNode>(Predicate<AstNode> predicate) {
    // TODO(brianwilkerson) It is a bug that this method can return `this`.
    AstNode node = this;
    while (node != null && !predicate(node)) {
      node = node.parent;
    }
    return node as E;
  }

  @override
  T thisOrAncestorOfType<T extends AstNode>() {
    AstNode node = this;
    while (node != null && node is! T) {
      node = node.parent;
    }
    return node as T;
  }

  @override
  String toSource() {
    StringBuffer buffer = new StringBuffer();
    accept(new ToSourceVisitor2(buffer));
    return buffer.toString();
  }

  @override
  String toString() => toSource();

  /**
   * Make this node the parent of the given [child] node. Return the child node.
   */
  T _becomeParentOf<T extends AstNodeImpl>(T child) {
    if (child != null) {
      child._parent = this;
    }
    return child;
  }
}

/**
 * An await expression.
 *
 *    awaitExpression ::=
 *        'await' [Expression]
 */
class AwaitExpressionImpl extends ExpressionImpl implements AwaitExpression {
  /**
   * The 'await' keyword.
   */
  @override
  Token awaitKeyword;

  /**
   * The expression whose value is being waited on.
   */
  ExpressionImpl _expression;

  /**
   * Initialize a newly created await expression.
   */
  AwaitExpressionImpl(this.awaitKeyword, ExpressionImpl expression) {
    _expression = _becomeParentOf(expression);
  }

  @override
  Token get beginToken {
    if (awaitKeyword != null) {
      return awaitKeyword;
    }
    return _expression.beginToken;
  }

  @override
  Iterable<SyntacticEntity> get childEntities =>
      new ChildEntities()..add(awaitKeyword)..add(_expression);

  @override
  Token get endToken => _expression.endToken;

  @override
  Expression get expression => _expression;

  @override
  void set expression(Expression expression) {
    _expression = _becomeParentOf(expression as ExpressionImpl);
  }

  @override
  int get precedence => 14;

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitAwaitExpression(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _expression?.accept(visitor);
  }
}

/**
 * A binary (infix) expression.
 *
 *    binaryExpression ::=
 *        [Expression] [Token] [Expression]
 */
class BinaryExpressionImpl extends ExpressionImpl implements BinaryExpression {
  /**
   * The expression used to compute the left operand.
   */
  ExpressionImpl _leftOperand;

  /**
   * The binary operator being applied.
   */
  @override
  Token operator;

  /**
   * The expression used to compute the right operand.
   */
  ExpressionImpl _rightOperand;

  /**
   * The element associated with the operator based on the static type of the
   * left operand, or `null` if the AST structure has not been resolved, if the
   * operator is not user definable, or if the operator could not be resolved.
   */
  @override
  MethodElement staticElement;

  @override
  FunctionType staticInvokeType;

  /**
   * Initialize a newly created binary expression.
   */
  BinaryExpressionImpl(
      ExpressionImpl leftOperand, this.operator, ExpressionImpl rightOperand) {
    _leftOperand = _becomeParentOf(leftOperand);
    _rightOperand = _becomeParentOf(rightOperand);
  }

  @override
  Token get beginToken => _leftOperand.beginToken;

  @override
  @deprecated
  MethodElement get bestElement => staticElement;

  @override
  Iterable<SyntacticEntity> get childEntities =>
      new ChildEntities()..add(_leftOperand)..add(operator)..add(_rightOperand);

  @override
  Token get endToken => _rightOperand.endToken;

  @override
  Expression get leftOperand => _leftOperand;

  @override
  void set leftOperand(Expression expression) {
    _leftOperand = _becomeParentOf(expression as ExpressionImpl);
  }

  @override
  int get precedence => operator.type.precedence;

  @deprecated
  @override
  MethodElement get propagatedElement => null;

  @deprecated
  @override
  set propagatedElement(MethodElement element) {}

  @override
  Expression get rightOperand => _rightOperand;

  @override
  void set rightOperand(Expression expression) {
    _rightOperand = _becomeParentOf(expression as ExpressionImpl);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitBinaryExpression(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _leftOperand?.accept(visitor);
    _rightOperand?.accept(visitor);
  }
}

/**
 * A function body that consists of a block of statements.
 *
 *    blockFunctionBody ::=
 *        ('async' | 'async' '*' | 'sync' '*')? [Block]
 */
class BlockFunctionBodyImpl extends FunctionBodyImpl
    implements BlockFunctionBody {
  /**
   * The token representing the 'async' or 'sync' keyword, or `null` if there is
   * no such keyword.
   */
  @override
  Token keyword;

  /**
   * The star optionally following the 'async' or 'sync' keyword, or `null` if
   * there is wither no such keyword or no star.
   */
  @override
  Token star;

  /**
   * The block representing the body of the function.
   */
  BlockImpl _block;

  /**
   * Initialize a newly created function body consisting of a block of
   * statements. The [keyword] can be `null` if there is no keyword specified
   * for the block. The [star] can be `null` if there is no star following the
   * keyword (and must be `null` if there is no keyword).
   */
  BlockFunctionBodyImpl(this.keyword, this.star, BlockImpl block) {
    _block = _becomeParentOf(block);
  }

  @override
  Token get beginToken {
    if (keyword != null) {
      return keyword;
    }
    return _block.beginToken;
  }

  @override
  Block get block => _block;

  @override
  void set block(Block block) {
    _block = _becomeParentOf(block as BlockImpl);
  }

  @override
  Iterable<SyntacticEntity> get childEntities =>
      new ChildEntities()..add(keyword)..add(star)..add(_block);

  @override
  Token get endToken => _block.endToken;

  @override
  bool get isAsynchronous => keyword != null && keyword.lexeme == Parser.ASYNC;

  @override
  bool get isGenerator => star != null;

  @override
  bool get isSynchronous => keyword == null || keyword.lexeme != Parser.ASYNC;

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitBlockFunctionBody(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _block?.accept(visitor);
  }
}

/**
 * A sequence of statements.
 *
 *    block ::=
 *        '{' statement* '}'
 */
class BlockImpl extends StatementImpl implements Block {
  /**
   * The left curly bracket.
   */
  @override
  Token leftBracket;

  /**
   * The statements contained in the block.
   */
  NodeList<Statement> _statements;

  /**
   * The right curly bracket.
   */
  @override
  Token rightBracket;

  /**
   * Initialize a newly created block of code.
   */
  BlockImpl(this.leftBracket, List<Statement> statements, this.rightBracket) {
    _statements = new NodeListImpl<Statement>(this, statements);
  }

  @override
  Token get beginToken => leftBracket;

  @override
  Iterable<SyntacticEntity> get childEntities => new ChildEntities()
    ..add(leftBracket)
    ..addAll(_statements)
    ..add(rightBracket);

  @override
  Token get endToken => rightBracket;

  @override
  NodeList<Statement> get statements => _statements;

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitBlock(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _statements.accept(visitor);
  }
}

/**
 * A boolean literal expression.
 *
 *    booleanLiteral ::=
 *        'false' | 'true'
 */
class BooleanLiteralImpl extends LiteralImpl implements BooleanLiteral {
  /**
   * The token representing the literal.
   */
  @override
  Token literal;

  /**
   * The value of the literal.
   */
  @override
  bool value = false;

  /**
   * Initialize a newly created boolean literal.
   */
  BooleanLiteralImpl(this.literal, this.value);

  @override
  Token get beginToken => literal;

  @override
  Iterable<SyntacticEntity> get childEntities =>
      new ChildEntities()..add(literal);

  @override
  Token get endToken => literal;

  @override
  bool get isSynthetic => literal.isSynthetic;

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitBooleanLiteral(this);

  @override
  void visitChildren(AstVisitor visitor) {
    // There are no children to visit.
  }
}

/**
 * A break statement.
 *
 *    breakStatement ::=
 *        'break' [SimpleIdentifier]? ';'
 */
class BreakStatementImpl extends StatementImpl implements BreakStatement {
  /**
   * The token representing the 'break' keyword.
   */
  @override
  Token breakKeyword;

  /**
   * The label associated with the statement, or `null` if there is no label.
   */
  SimpleIdentifierImpl _label;

  /**
   * The semicolon terminating the statement.
   */
  @override
  Token semicolon;

  /**
   * The AstNode which this break statement is breaking from.  This will be
   * either a [Statement] (in the case of breaking out of a loop), a
   * [SwitchMember] (in the case of a labeled break statement whose label
   * matches a label on a switch case in an enclosing switch statement), or
   * `null` if the AST has not yet been resolved or if the target could not be
   * resolved. Note that if the source code has errors, the target might be
   * invalid (e.g. trying to break to a switch case).
   */
  @override
  AstNode target;

  /**
   * Initialize a newly created break statement. The [label] can be `null` if
   * there is no label associated with the statement.
   */
  BreakStatementImpl(
      this.breakKeyword, SimpleIdentifierImpl label, this.semicolon) {
    _label = _becomeParentOf(label);
  }

  @override
  Token get beginToken => breakKeyword;

  @override
  Iterable<SyntacticEntity> get childEntities =>
      new ChildEntities()..add(breakKeyword)..add(_label)..add(semicolon);

  @override
  Token get endToken => semicolon;

  @override
  SimpleIdentifier get label => _label;

  @override
  void set label(SimpleIdentifier identifier) {
    _label = _becomeParentOf(identifier as SimpleIdentifierImpl);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitBreakStatement(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _label?.accept(visitor);
  }
}

/**
 * A sequence of cascaded expressions: expressions that share a common target.
 * There are three kinds of expressions that can be used in a cascade
 * expression: [IndexExpression], [MethodInvocation] and [PropertyAccess].
 *
 *    cascadeExpression ::=
 *        [Expression] cascadeSection*
 *
 *    cascadeSection ::=
 *        '..'  (cascadeSelector arguments*) (assignableSelector arguments*)*
 *        (assignmentOperator expressionWithoutCascade)?
 *
 *    cascadeSelector ::=
 *        '[ ' expression '] '
 *      | identifier
 */
class CascadeExpressionImpl extends ExpressionImpl
    implements CascadeExpression {
  /**
   * The target of the cascade sections.
   */
  ExpressionImpl _target;

  /**
   * The cascade sections sharing the common target.
   */
  NodeList<Expression> _cascadeSections;

  /**
   * Initialize a newly created cascade expression. The list of
   * [cascadeSections] must contain at least one element.
   */
  CascadeExpressionImpl(
      ExpressionImpl target, List<Expression> cascadeSections) {
    _target = _becomeParentOf(target);
    _cascadeSections = new NodeListImpl<Expression>(this, cascadeSections);
  }

  @override
  Token get beginToken => _target.beginToken;

  @override
  NodeList<Expression> get cascadeSections => _cascadeSections;

  @override
  Iterable<SyntacticEntity> get childEntities => new ChildEntities()
    ..add(_target)
    ..addAll(_cascadeSections);

  @override
  Token get endToken => _cascadeSections.endToken;

  @override
  int get precedence => 2;

  @override
  Expression get target => _target;

  @override
  void set target(Expression target) {
    _target = _becomeParentOf(target as ExpressionImpl);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitCascadeExpression(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _target?.accept(visitor);
    _cascadeSections.accept(visitor);
  }
}

/**
 * A catch clause within a try statement.
 *
 *    onPart ::=
 *        catchPart [Block]
 *      | 'on' type catchPart? [Block]
 *
 *    catchPart ::=
 *        'catch' '(' [SimpleIdentifier] (',' [SimpleIdentifier])? ')'
 */
class CatchClauseImpl extends AstNodeImpl implements CatchClause {
  /**
   * The token representing the 'on' keyword, or `null` if there is no 'on'
   * keyword.
   */
  @override
  Token onKeyword;

  /**
   * The type of exceptions caught by this catch clause, or `null` if this catch
   * clause catches every type of exception.
   */
  TypeAnnotationImpl _exceptionType;

  /**
   * The token representing the 'catch' keyword, or `null` if there is no
   * 'catch' keyword.
   */
  @override
  Token catchKeyword;

  /**
   * The left parenthesis, or `null` if there is no 'catch' keyword.
   */
  @override
  Token leftParenthesis;

  /**
   * The parameter whose value will be the exception that was thrown, or `null`
   * if there is no 'catch' keyword.
   */
  SimpleIdentifierImpl _exceptionParameter;

  /**
   * The comma separating the exception parameter from the stack trace
   * parameter, or `null` if there is no stack trace parameter.
   */
  @override
  Token comma;

  /**
   * The parameter whose value will be the stack trace associated with the
   * exception, or `null` if there is no stack trace parameter.
   */
  SimpleIdentifierImpl _stackTraceParameter;

  /**
   * The right parenthesis, or `null` if there is no 'catch' keyword.
   */
  @override
  Token rightParenthesis;

  /**
   * The body of the catch block.
   */
  BlockImpl _body;

  /**
   * Initialize a newly created catch clause. The [onKeyword] and
   * [exceptionType] can be `null` if the clause will catch all exceptions. The
   * [comma] and [stackTraceParameter] can be `null` if the stack trace
   * parameter is not defined.
   */
  CatchClauseImpl(
      this.onKeyword,
      TypeAnnotationImpl exceptionType,
      this.catchKeyword,
      this.leftParenthesis,
      SimpleIdentifierImpl exceptionParameter,
      this.comma,
      SimpleIdentifierImpl stackTraceParameter,
      this.rightParenthesis,
      BlockImpl body) {
    _exceptionType = _becomeParentOf(exceptionType);
    _exceptionParameter = _becomeParentOf(exceptionParameter);
    _stackTraceParameter = _becomeParentOf(stackTraceParameter);
    _body = _becomeParentOf(body);
  }

  @override
  Token get beginToken {
    if (onKeyword != null) {
      return onKeyword;
    }
    return catchKeyword;
  }

  @override
  Block get body => _body;

  @override
  void set body(Block block) {
    _body = _becomeParentOf(block as BlockImpl);
  }

  @override
  Iterable<SyntacticEntity> get childEntities => new ChildEntities()
    ..add(onKeyword)
    ..add(_exceptionType)
    ..add(catchKeyword)
    ..add(leftParenthesis)
    ..add(_exceptionParameter)
    ..add(comma)
    ..add(_stackTraceParameter)
    ..add(rightParenthesis)
    ..add(_body);

  @override
  Token get endToken => _body.endToken;

  @override
  SimpleIdentifier get exceptionParameter => _exceptionParameter;

  @override
  void set exceptionParameter(SimpleIdentifier parameter) {
    _exceptionParameter = _becomeParentOf(parameter as SimpleIdentifierImpl);
  }

  @override
  TypeAnnotation get exceptionType => _exceptionType;

  @override
  void set exceptionType(TypeAnnotation exceptionType) {
    _exceptionType = _becomeParentOf(exceptionType as TypeAnnotationImpl);
  }

  @override
  SimpleIdentifier get stackTraceParameter => _stackTraceParameter;

  @override
  void set stackTraceParameter(SimpleIdentifier parameter) {
    _stackTraceParameter = _becomeParentOf(parameter as SimpleIdentifierImpl);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitCatchClause(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _exceptionType?.accept(visitor);
    _exceptionParameter?.accept(visitor);
    _stackTraceParameter?.accept(visitor);
    _body?.accept(visitor);
  }
}

/**
 * Helper class to allow iteration of child entities of an AST node.
 */
class ChildEntities
    with IterableMixin<SyntacticEntity>
    implements Iterable<SyntacticEntity> {
  /**
   * The list of child entities to be iterated over.
   */
  List<SyntacticEntity> _entities = [];

  @override
  Iterator<SyntacticEntity> get iterator => _entities.iterator;

  /**
   * Add an AST node or token as the next child entity, if it is not null.
   */
  void add(SyntacticEntity entity) {
    if (entity != null) {
      _entities.add(entity);
    }
  }

  /**
   * Add the given items as the next child entities, if [items] is not null.
   */
  void addAll(Iterable<SyntacticEntity> items) {
    if (items != null) {
      _entities.addAll(items);
    }
  }
}

/**
 * The declaration of a class.
 *
 *    classDeclaration ::=
 *        'abstract'? 'class' [SimpleIdentifier] [TypeParameterList]?
 *        ([ExtendsClause] [WithClause]?)?
 *        [ImplementsClause]?
 *        '{' [ClassMember]* '}'
 */
class ClassDeclarationImpl extends ClassOrMixinDeclarationImpl
    implements ClassDeclaration {
  /**
   * The 'abstract' keyword, or `null` if the keyword was absent.
   */
  @override
  Token abstractKeyword;

  /**
   * The token representing the 'class' keyword.
   */
  @override
  Token classKeyword;

  /**
   * The extends clause for the class, or `null` if the class does not extend
   * any other class.
   */
  ExtendsClauseImpl _extendsClause;

  /**
   * The with clause for the class, or `null` if the class does not have a with
   * clause.
   */
  WithClauseImpl _withClause;

  /**
   * The native clause for the class, or `null` if the class does not have a
   * native clause.
   */
  NativeClauseImpl _nativeClause;

  /**
   * Initialize a newly created class declaration. Either or both of the
   * [comment] and [metadata] can be `null` if the class does not have the
   * corresponding attribute. The [abstractKeyword] can be `null` if the class
   * is not abstract. The [typeParameters] can be `null` if the class does not
   * have any type parameters. Any or all of the [extendsClause], [withClause],
   * and [implementsClause] can be `null` if the class does not have the
   * corresponding clause. The list of [members] can be `null` if the class does
   * not have any members.
   */
  ClassDeclarationImpl(
      CommentImpl comment,
      List<Annotation> metadata,
      this.abstractKeyword,
      this.classKeyword,
      SimpleIdentifierImpl name,
      TypeParameterListImpl typeParameters,
      ExtendsClauseImpl extendsClause,
      WithClauseImpl withClause,
      ImplementsClauseImpl implementsClause,
      Token leftBracket,
      List<ClassMember> members,
      Token rightBracket)
      : super(comment, metadata, name, typeParameters, implementsClause,
            leftBracket, members, rightBracket) {
    _extendsClause = _becomeParentOf(extendsClause);
    _withClause = _becomeParentOf(withClause);
  }

  @override
  Iterable<SyntacticEntity> get childEntities => super._childEntities
    ..add(abstractKeyword)
    ..add(classKeyword)
    ..add(_name)
    ..add(_typeParameters)
    ..add(_extendsClause)
    ..add(_withClause)
    ..add(_implementsClause)
    ..add(_nativeClause)
    ..add(leftBracket)
    ..addAll(members)
    ..add(rightBracket);

  @override
  ClassElement get declaredElement => _name?.staticElement as ClassElement;

  @deprecated
  @override
  ClassElement get element => declaredElement;

  @override
  ExtendsClause get extendsClause => _extendsClause;

  @override
  void set extendsClause(ExtendsClause extendsClause) {
    _extendsClause = _becomeParentOf(extendsClause as ExtendsClauseImpl);
  }

  @override
  Token get firstTokenAfterCommentAndMetadata {
    if (abstractKeyword != null) {
      return abstractKeyword;
    }
    return classKeyword;
  }

  @override
  bool get isAbstract => abstractKeyword != null;

  @override
  NativeClause get nativeClause => _nativeClause;

  @override
  void set nativeClause(NativeClause nativeClause) {
    _nativeClause = _becomeParentOf(nativeClause as NativeClauseImpl);
  }

  @override
  WithClause get withClause => _withClause;

  @override
  void set withClause(WithClause withClause) {
    _withClause = _becomeParentOf(withClause as WithClauseImpl);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitClassDeclaration(this);

  @override
  ConstructorDeclaration getConstructor(String name) {
    int length = _members.length;
    for (int i = 0; i < length; i++) {
      ClassMember classMember = _members[i];
      if (classMember is ConstructorDeclaration) {
        ConstructorDeclaration constructor = classMember;
        SimpleIdentifier constructorName = constructor.name;
        if (name == null && constructorName == null) {
          return constructor;
        }
        if (constructorName != null && constructorName.name == name) {
          return constructor;
        }
      }
    }
    return null;
  }

  @override
  void visitChildren(AstVisitor visitor) {
    super.visitChildren(visitor);
    _name?.accept(visitor);
    _typeParameters?.accept(visitor);
    _extendsClause?.accept(visitor);
    _withClause?.accept(visitor);
    _implementsClause?.accept(visitor);
    _nativeClause?.accept(visitor);
    members.accept(visitor);
  }
}

/**
 * A node that declares a name within the scope of a class.
 */
abstract class ClassMemberImpl extends DeclarationImpl implements ClassMember {
  /**
   * Initialize a newly created member of a class. Either or both of the
   * [comment] and [metadata] can be `null` if the member does not have the
   * corresponding attribute.
   */
  ClassMemberImpl(CommentImpl comment, List<Annotation> metadata)
      : super(comment, metadata);
}

abstract class ClassOrMixinDeclarationImpl
    extends NamedCompilationUnitMemberImpl implements ClassOrMixinDeclaration {
  /**
   * The type parameters for the class or mixin,
   * or `null` if the declaration does not have any type parameters.
   */
  TypeParameterListImpl _typeParameters;

  /**
   * The implements clause for the class or mixin,
   * or `null` if the declaration does not implement any interfaces.
   */
  ImplementsClauseImpl _implementsClause;

  /**
   * The left curly bracket.
   */
  Token leftBracket;

  /**
   * The members defined by the class or mixin.
   */
  NodeList<ClassMember> _members;

  /**
   * The right curly bracket.
   */
  Token rightBracket;

  ClassOrMixinDeclarationImpl(
      CommentImpl comment,
      List<Annotation> metadata,
      SimpleIdentifierImpl name,
      TypeParameterListImpl typeParameters,
      ImplementsClauseImpl implementsClause,
      this.leftBracket,
      List<ClassMember> members,
      this.rightBracket)
      : super(comment, metadata, name) {
    _typeParameters = _becomeParentOf(typeParameters);
    _implementsClause = _becomeParentOf(implementsClause);
    _members = new NodeListImpl<ClassMember>(this, members);
  }

  Token get endToken => rightBracket;

  ImplementsClause get implementsClause => _implementsClause;

  void set implementsClause(ImplementsClause implementsClause) {
    _implementsClause =
        _becomeParentOf(implementsClause as ImplementsClauseImpl);
  }

  NodeList<ClassMember> get members => _members;

  TypeParameterList get typeParameters => _typeParameters;

  void set typeParameters(TypeParameterList typeParameters) {
    _typeParameters = _becomeParentOf(typeParameters as TypeParameterListImpl);
  }

  VariableDeclaration getField(String name) {
    int memberLength = _members.length;
    for (int i = 0; i < memberLength; i++) {
      ClassMember classMember = _members[i];
      if (classMember is FieldDeclaration) {
        FieldDeclaration fieldDeclaration = classMember;
        NodeList<VariableDeclaration> fields =
            fieldDeclaration.fields.variables;
        int fieldLength = fields.length;
        for (int i = 0; i < fieldLength; i++) {
          VariableDeclaration field = fields[i];
          SimpleIdentifier fieldName = field.name;
          if (fieldName != null && name == fieldName.name) {
            return field;
          }
        }
      }
    }
    return null;
  }

  MethodDeclaration getMethod(String name) {
    int length = _members.length;
    for (int i = 0; i < length; i++) {
      ClassMember classMember = _members[i];
      if (classMember is MethodDeclaration) {
        MethodDeclaration method = classMember;
        SimpleIdentifier methodName = method.name;
        if (methodName != null && name == methodName.name) {
          return method;
        }
      }
    }
    return null;
  }
}

/**
 * A class type alias.
 *
 *    classTypeAlias ::=
 *        [SimpleIdentifier] [TypeParameterList]? '=' 'abstract'? mixinApplication
 *
 *    mixinApplication ::=
 *        [TypeName] [WithClause] [ImplementsClause]? ';'
 */
class ClassTypeAliasImpl extends TypeAliasImpl implements ClassTypeAlias {
  /**
   * The type parameters for the class, or `null` if the class does not have any
   * type parameters.
   */
  TypeParameterListImpl _typeParameters;

  /**
   * The token for the '=' separating the name from the definition.
   */
  @override
  Token equals;

  /**
   * The token for the 'abstract' keyword, or `null` if this is not defining an
   * abstract class.
   */
  @override
  Token abstractKeyword;

  /**
   * The name of the superclass of the class being declared.
   */
  TypeNameImpl _superclass;

  /**
   * The with clause for this class.
   */
  WithClauseImpl _withClause;

  /**
   * The implements clause for this class, or `null` if there is no implements
   * clause.
   */
  ImplementsClauseImpl _implementsClause;

  /**
   * Initialize a newly created class type alias. Either or both of the
   * [comment] and [metadata] can be `null` if the class type alias does not
   * have the corresponding attribute. The [typeParameters] can be `null` if the
   * class does not have any type parameters. The [abstractKeyword] can be
   * `null` if the class is not abstract. The [implementsClause] can be `null`
   * if the class does not implement any interfaces.
   */
  ClassTypeAliasImpl(
      CommentImpl comment,
      List<Annotation> metadata,
      Token keyword,
      SimpleIdentifierImpl name,
      TypeParameterListImpl typeParameters,
      this.equals,
      this.abstractKeyword,
      TypeNameImpl superclass,
      WithClauseImpl withClause,
      ImplementsClauseImpl implementsClause,
      Token semicolon)
      : super(comment, metadata, keyword, name, semicolon) {
    _typeParameters = _becomeParentOf(typeParameters);
    _superclass = _becomeParentOf(superclass);
    _withClause = _becomeParentOf(withClause);
    _implementsClause = _becomeParentOf(implementsClause);
  }

  @override
  Iterable<SyntacticEntity> get childEntities => super._childEntities
    ..add(typedefKeyword)
    ..add(_name)
    ..add(_typeParameters)
    ..add(equals)
    ..add(abstractKeyword)
    ..add(_superclass)
    ..add(_withClause)
    ..add(_implementsClause)
    ..add(semicolon);

  @override
  ClassElement get declaredElement => _name?.staticElement as ClassElement;

  @deprecated
  @override
  ClassElement get element => declaredElement;

  @override
  Token get firstTokenAfterCommentAndMetadata {
    if (abstractKeyword != null) {
      return abstractKeyword;
    }
    return typedefKeyword;
  }

  @override
  ImplementsClause get implementsClause => _implementsClause;

  @override
  void set implementsClause(ImplementsClause implementsClause) {
    _implementsClause =
        _becomeParentOf(implementsClause as ImplementsClauseImpl);
  }

  @override
  bool get isAbstract => abstractKeyword != null;

  @override
  TypeName get superclass => _superclass;

  @override
  void set superclass(TypeName superclass) {
    _superclass = _becomeParentOf(superclass as TypeNameImpl);
  }

  @override
  TypeParameterList get typeParameters => _typeParameters;

  @override
  void set typeParameters(TypeParameterList typeParameters) {
    _typeParameters = _becomeParentOf(typeParameters as TypeParameterListImpl);
  }

  @override
  WithClause get withClause => _withClause;

  @override
  void set withClause(WithClause withClause) {
    _withClause = _becomeParentOf(withClause as WithClauseImpl);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitClassTypeAlias(this);

  @override
  void visitChildren(AstVisitor visitor) {
    super.visitChildren(visitor);
    _name?.accept(visitor);
    _typeParameters?.accept(visitor);
    _superclass?.accept(visitor);
    _withClause?.accept(visitor);
    _implementsClause?.accept(visitor);
  }
}

abstract class CollectionElementImpl extends AstNodeImpl
    implements CollectionElement {}

class CollectionForElementImpl extends CollectionElementImpl
    with ForMixin
    implements CollectionForElement {
  /**
   * The body of the loop.
   */
  CollectionElementImpl _body;

  /**
   * Initialize a newly created for element.
   */
  CollectionForElementImpl(
      Token awaitKeyword,
      Token forKeyword,
      Token leftParenthesis,
      ForLoopPartsImpl forLoopParts,
      Token rightParenthesis,
      CollectionElementImpl body) {
    this.awaitKeyword = awaitKeyword;
    this.forKeyword = forKeyword;
    this.leftParenthesis = leftParenthesis;
    _forLoopParts = _becomeParentOf(forLoopParts);
    this.rightParenthesis = rightParenthesis;
    _body = _becomeParentOf(body);
  }

  @override
  CollectionElement get body => _body;

  void set body(CollectionElement statement) {
    _body = _becomeParentOf(statement as CollectionElementImpl);
  }

  @override
  Iterable<SyntacticEntity> get childEntities => new ChildEntities()
    ..addAll(super.childEntities)
    ..add(_body);

  @override
  Token get endToken => _body.endToken;

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitCollectionForElement(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _forLoopParts?.accept(visitor);
    _body?.accept(visitor);
  }
}

class CollectionIfElementImpl extends CollectionElementImpl
    with IfMixin
    implements CollectionIfElement {
  /**
   * The element to be executed if the condition is `true`.
   */
  CollectionElementImpl _thenElement;

  /**
   * The element to be executed if the condition is `false`, or `null` if there
   * is no such element.
   */
  CollectionElementImpl _elseElement;

  /**
   * Initialize a newly created for element.
   */
  CollectionIfElementImpl(
      Token ifKeyword,
      Token leftParenthesis,
      ExpressionImpl condition,
      Token rightParenthesis,
      CollectionElementImpl thenElement,
      Token elseKeyword,
      CollectionElementImpl elseElement) {
    this.ifKeyword = ifKeyword;
    this.leftParenthesis = leftParenthesis;
    _condition = _becomeParentOf(condition);
    this.rightParenthesis = rightParenthesis;
    _thenElement = _becomeParentOf(thenElement);
    this.elseKeyword = elseKeyword;
    _elseElement = _becomeParentOf(elseElement);
  }

  @override
  Iterable<SyntacticEntity> get childEntities => new ChildEntities()
    ..addAll(super.childEntities)
    ..add(_thenElement)
    ..add(_elseElement);

  @override
  CollectionElement get elseElement => _elseElement;

  set elseElement(CollectionElement element) {
    _elseElement = _becomeParentOf(element as CollectionElementImpl);
  }

  @override
  Token get endToken => _elseElement?.endToken ?? _thenElement.endToken;

  @override
  CollectionElement get thenElement => _thenElement;

  set thenElement(CollectionElement element) {
    _thenElement = _becomeParentOf(element as CollectionElementImpl);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitCollectionIfElement(this);

  @override
  void visitChildren(AstVisitor visitor) {
    super.visitChildren(visitor);
    _thenElement?.accept(visitor);
    _elseElement?.accept(visitor);
  }
}

/**
 * A combinator associated with an import or export directive.
 *
 *    combinator ::=
 *        [HideCombinator]
 *      | [ShowCombinator]
 */
abstract class CombinatorImpl extends AstNodeImpl implements Combinator {
  /**
   * The 'hide' or 'show' keyword specifying what kind of processing is to be
   * done on the names.
   */
  @override
  Token keyword;

  /**
   * Initialize a newly created combinator.
   */
  CombinatorImpl(this.keyword);

  @override
  Token get beginToken => keyword;
}

/**
 * A comment within the source code.
 *
 *    comment ::=
 *        endOfLineComment
 *      | blockComment
 *      | documentationComment
 *
 *    endOfLineComment ::=
 *        '//' (CHARACTER - EOL)* EOL
 *
 *    blockComment ::=
 *        '/ *' CHARACTER* '&#42;/'
 *
 *    documentationComment ::=
 *        '/ **' (CHARACTER | [CommentReference])* '&#42;/'
 *      | ('///' (CHARACTER - EOL)* EOL)+
 */
class CommentImpl extends AstNodeImpl implements Comment {
  /**
   * The tokens representing the comment.
   */
  @override
  final List<Token> tokens;

  /**
   * The type of the comment.
   */
  final CommentType _type;

  /**
   * The references embedded within the documentation comment. This list will be
   * empty unless this is a documentation comment that has references embedded
   * within it.
   */
  NodeList<CommentReference> _references;

  /**
   * Initialize a newly created comment. The list of [tokens] must contain at
   * least one token. The [_type] is the type of the comment. The list of
   * [references] can be empty if the comment does not contain any embedded
   * references.
   */
  CommentImpl(this.tokens, this._type, List<CommentReference> references) {
    _references = new NodeListImpl<CommentReference>(this, references);
  }

  @override
  Token get beginToken => tokens[0];

  @override
  Iterable<SyntacticEntity> get childEntities =>
      new ChildEntities()..addAll(tokens);

  @override
  Token get endToken => tokens[tokens.length - 1];

  @override
  bool get isBlock => _type == CommentType.BLOCK;

  @override
  bool get isDocumentation => _type == CommentType.DOCUMENTATION;

  @override
  bool get isEndOfLine => _type == CommentType.END_OF_LINE;

  @override
  NodeList<CommentReference> get references => _references;

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitComment(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _references.accept(visitor);
  }

  /**
   * Create a block comment consisting of the given [tokens].
   */
  static Comment createBlockComment(List<Token> tokens) =>
      new CommentImpl(tokens, CommentType.BLOCK, null);

  /**
   * Create a documentation comment consisting of the given [tokens].
   */
  static Comment createDocumentationComment(List<Token> tokens) =>
      new CommentImpl(
          tokens, CommentType.DOCUMENTATION, new List<CommentReference>());

  /**
   * Create a documentation comment consisting of the given [tokens] and having
   * the given [references] embedded within it.
   */
  static Comment createDocumentationCommentWithReferences(
          List<Token> tokens, List<CommentReference> references) =>
      new CommentImpl(tokens, CommentType.DOCUMENTATION, references);

  /**
   * Create an end-of-line comment consisting of the given [tokens].
   */
  static Comment createEndOfLineComment(List<Token> tokens) =>
      new CommentImpl(tokens, CommentType.END_OF_LINE, null);
}

/**
 * A reference to a Dart element that is found within a documentation comment.
 *
 *    commentReference ::=
 *        '[' 'new'? [Identifier] ']'
 */
class CommentReferenceImpl extends AstNodeImpl implements CommentReference {
  /**
   * The token representing the 'new' keyword, or `null` if there was no 'new'
   * keyword.
   */
  @override
  Token newKeyword;

  /**
   * The identifier being referenced.
   */
  IdentifierImpl _identifier;

  /**
   * Initialize a newly created reference to a Dart element. The [newKeyword]
   * can be `null` if the reference is not to a constructor.
   */
  CommentReferenceImpl(this.newKeyword, IdentifierImpl identifier) {
    _identifier = _becomeParentOf(identifier);
  }

  @override
  Token get beginToken => newKeyword ?? _identifier.beginToken;

  @override
  Iterable<SyntacticEntity> get childEntities =>
      new ChildEntities()..add(newKeyword)..add(_identifier);

  @override
  Token get endToken => _identifier.endToken;

  @override
  Identifier get identifier => _identifier;

  @override
  void set identifier(Identifier identifier) {
    _identifier = _becomeParentOf(identifier as IdentifierImpl);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitCommentReference(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _identifier?.accept(visitor);
  }
}

/**
 * The possible types of comments that are recognized by the parser.
 */
class CommentType {
  /**
   * A block comment.
   */
  static const CommentType BLOCK = const CommentType('BLOCK');

  /**
   * A documentation comment.
   */
  static const CommentType DOCUMENTATION = const CommentType('DOCUMENTATION');

  /**
   * An end-of-line comment.
   */
  static const CommentType END_OF_LINE = const CommentType('END_OF_LINE');

  /**
   * The name of the comment type.
   */
  final String name;

  /**
   * Initialize a newly created comment type to have the given [name].
   */
  const CommentType(this.name);

  @override
  String toString() => name;
}

/**
 * A compilation unit.
 *
 * While the grammar restricts the order of the directives and declarations
 * within a compilation unit, this class does not enforce those restrictions.
 * In particular, the children of a compilation unit will be visited in lexical
 * order even if lexical order does not conform to the restrictions of the
 * grammar.
 *
 *    compilationUnit ::=
 *        directives declarations
 *
 *    directives ::=
 *        [ScriptTag]? [LibraryDirective]? namespaceDirective* [PartDirective]*
 *      | [PartOfDirective]
 *
 *    namespaceDirective ::=
 *        [ImportDirective]
 *      | [ExportDirective]
 *
 *    declarations ::=
 *        [CompilationUnitMember]*
 */
class CompilationUnitImpl extends AstNodeImpl implements CompilationUnit {
  /**
   * The first token in the token stream that was parsed to form this
   * compilation unit.
   */
  @override
  Token beginToken;

  /**
   * The script tag at the beginning of the compilation unit, or `null` if there
   * is no script tag in this compilation unit.
   */
  ScriptTagImpl _scriptTag;

  /**
   * The directives contained in this compilation unit.
   */
  NodeList<Directive> _directives;

  /**
   * The declarations contained in this compilation unit.
   */
  NodeList<CompilationUnitMember> _declarations;

  /**
   * The last token in the token stream that was parsed to form this compilation
   * unit. This token should always have a type of [TokenType.EOF].
   */
  @override
  Token endToken;

  /**
   * The element associated with this compilation unit, or `null` if the AST
   * structure has not been resolved.
   */
  @override
  CompilationUnitElement declaredElement;

  /**
   * The line information for this compilation unit.
   */
  @override
  LineInfo lineInfo;

  Map<int, AstNode> localDeclarations;

  /**
   * Is `true` if the non-nullable feature is enabled, and this library
   * unit is annotated with `@pragma('analyzer:non-nullable')`.
   */
  bool hasPragmaAnalyzerNonNullable = false;

  /**
   * Initialize a newly created compilation unit to have the given directives
   * and declarations. The [scriptTag] can be `null` if there is no script tag
   * in the compilation unit. The list of [directives] can be `null` if there
   * are no directives in the compilation unit. The list of [declarations] can
   * be `null` if there are no declarations in the compilation unit.
   */
  CompilationUnitImpl(
      this.beginToken,
      ScriptTagImpl scriptTag,
      List<Directive> directives,
      List<CompilationUnitMember> declarations,
      this.endToken) {
    _scriptTag = _becomeParentOf(scriptTag);
    _directives = new NodeListImpl<Directive>(this, directives);
    _declarations = new NodeListImpl<CompilationUnitMember>(this, declarations);
  }

  @override
  Iterable<SyntacticEntity> get childEntities {
    ChildEntities result = new ChildEntities()..add(_scriptTag);
    if (_directivesAreBeforeDeclarations) {
      result..addAll(_directives)..addAll(_declarations);
    } else {
      result.addAll(sortedDirectivesAndDeclarations);
    }
    return result;
  }

  @override
  NodeList<CompilationUnitMember> get declarations => _declarations;

  @override
  NodeList<Directive> get directives => _directives;

  @deprecated
  @override
  CompilationUnitElement get element => declaredElement;

  @override
  set element(CompilationUnitElement element) {
    declaredElement = element;
  }

  @override
  int get length {
    Token endToken = this.endToken;
    if (endToken == null) {
      return 0;
    }
    return endToken.offset + endToken.length;
  }

  @override
  int get offset => 0;

  @override
  ScriptTag get scriptTag => _scriptTag;

  @override
  void set scriptTag(ScriptTag scriptTag) {
    _scriptTag = _becomeParentOf(scriptTag as ScriptTagImpl);
  }

  @override
  List<AstNode> get sortedDirectivesAndDeclarations {
    return <AstNode>[]
      ..addAll(_directives)
      ..addAll(_declarations)
      ..sort(AstNode.LEXICAL_ORDER);
  }

  /**
   * Return `true` if all of the directives are lexically before any
   * declarations.
   */
  bool get _directivesAreBeforeDeclarations {
    if (_directives.isEmpty || _declarations.isEmpty) {
      return true;
    }
    Directive lastDirective = _directives[_directives.length - 1];
    CompilationUnitMember firstDeclaration = _declarations[0];
    return lastDirective.offset < firstDeclaration.offset;
  }

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitCompilationUnit(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _scriptTag?.accept(visitor);
    if (_directivesAreBeforeDeclarations) {
      _directives.accept(visitor);
      _declarations.accept(visitor);
    } else {
      List<AstNode> sortedMembers = sortedDirectivesAndDeclarations;
      int length = sortedMembers.length;
      for (int i = 0; i < length; i++) {
        AstNode child = sortedMembers[i];
        child.accept(visitor);
      }
    }
  }
}

/**
 * A node that declares one or more names within the scope of a compilation
 * unit.
 *
 *    compilationUnitMember ::=
 *        [ClassDeclaration]
 *      | [TypeAlias]
 *      | [FunctionDeclaration]
 *      | [MethodDeclaration]
 *      | [VariableDeclaration]
 *      | [VariableDeclaration]
 */
abstract class CompilationUnitMemberImpl extends DeclarationImpl
    implements CompilationUnitMember {
  /**
   * Initialize a newly created generic compilation unit member. Either or both
   * of the [comment] and [metadata] can be `null` if the member does not have
   * the corresponding attribute.
   */
  CompilationUnitMemberImpl(CommentImpl comment, List<Annotation> metadata)
      : super(comment, metadata);
}

/**
 * A conditional expression.
 *
 *    conditionalExpression ::=
 *        [Expression] '?' [Expression] ':' [Expression]
 */
class ConditionalExpressionImpl extends ExpressionImpl
    implements ConditionalExpression {
  /**
   * The condition used to determine which of the expressions is executed next.
   */
  ExpressionImpl _condition;

  /**
   * The token used to separate the condition from the then expression.
   */
  @override
  Token question;

  /**
   * The expression that is executed if the condition evaluates to `true`.
   */
  ExpressionImpl _thenExpression;

  /**
   * The token used to separate the then expression from the else expression.
   */
  @override
  Token colon;

  /**
   * The expression that is executed if the condition evaluates to `false`.
   */
  ExpressionImpl _elseExpression;

  /**
   * Initialize a newly created conditional expression.
   */
  ConditionalExpressionImpl(
      ExpressionImpl condition,
      this.question,
      ExpressionImpl thenExpression,
      this.colon,
      ExpressionImpl elseExpression) {
    _condition = _becomeParentOf(condition);
    _thenExpression = _becomeParentOf(thenExpression);
    _elseExpression = _becomeParentOf(elseExpression);
  }

  @override
  Token get beginToken => _condition.beginToken;

  @override
  Iterable<SyntacticEntity> get childEntities => new ChildEntities()
    ..add(_condition)
    ..add(question)
    ..add(_thenExpression)
    ..add(colon)
    ..add(_elseExpression);

  @override
  Expression get condition => _condition;

  @override
  void set condition(Expression expression) {
    _condition = _becomeParentOf(expression as ExpressionImpl);
  }

  @override
  Expression get elseExpression => _elseExpression;

  @override
  void set elseExpression(Expression expression) {
    _elseExpression = _becomeParentOf(expression as ExpressionImpl);
  }

  @override
  Token get endToken => _elseExpression.endToken;

  @override
  int get precedence => 3;

  @override
  Expression get thenExpression => _thenExpression;

  @override
  void set thenExpression(Expression expression) {
    _thenExpression = _becomeParentOf(expression as ExpressionImpl);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) =>
      visitor.visitConditionalExpression(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _condition?.accept(visitor);
    _thenExpression?.accept(visitor);
    _elseExpression?.accept(visitor);
  }
}

/**
 * A configuration in either an import or export directive.
 *
 *     configuration ::=
 *         'if' '(' test ')' uri
 *
 *     test ::=
 *         dottedName ('==' stringLiteral)?
 *
 *     dottedName ::=
 *         identifier ('.' identifier)*
 */
class ConfigurationImpl extends AstNodeImpl implements Configuration {
  @override
  Token ifKeyword;

  @override
  Token leftParenthesis;

  DottedNameImpl _name;

  @override
  Token equalToken;

  StringLiteralImpl _value;

  @override
  Token rightParenthesis;

  StringLiteralImpl _uri;

  @override
  Source uriSource;

  ConfigurationImpl(
      this.ifKeyword,
      this.leftParenthesis,
      DottedNameImpl name,
      this.equalToken,
      StringLiteralImpl value,
      this.rightParenthesis,
      StringLiteralImpl libraryUri) {
    _name = _becomeParentOf(name);
    _value = _becomeParentOf(value);
    _uri = _becomeParentOf(libraryUri);
  }

  @override
  Token get beginToken => ifKeyword;

  @override
  Iterable<SyntacticEntity> get childEntities => new ChildEntities()
    ..add(ifKeyword)
    ..add(leftParenthesis)
    ..add(_name)
    ..add(equalToken)
    ..add(_value)
    ..add(rightParenthesis)
    ..add(_uri);

  @override
  Token get endToken => _uri.endToken;

  @deprecated
  @override
  StringLiteral get libraryUri => _uri;

  @deprecated
  @override
  void set libraryUri(StringLiteral libraryUri) {
    _uri = _becomeParentOf(libraryUri as StringLiteralImpl);
  }

  @override
  DottedName get name => _name;

  @override
  void set name(DottedName name) {
    _name = _becomeParentOf(name as DottedNameImpl);
  }

  @override
  StringLiteral get uri => _uri;

  @override
  void set uri(StringLiteral uri) {
    _uri = _becomeParentOf(uri as StringLiteralImpl);
  }

  @override
  StringLiteral get value => _value;

  @override
  void set value(StringLiteral value) {
    _value = _becomeParentOf(value as StringLiteralImpl);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitConfiguration(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _name?.accept(visitor);
    _value?.accept(visitor);
    _uri?.accept(visitor);
  }
}

/**
 * An error listener that only records whether any constant related errors have
 * been reported.
 */
class ConstantAnalysisErrorListener extends AnalysisErrorListener {
  /**
   * A flag indicating whether any constant related errors have been reported to
   * this listener.
   */
  bool hasConstError = false;

  @override
  void onError(AnalysisError error) {
    ErrorCode errorCode = error.errorCode;
    if (errorCode is CompileTimeErrorCode) {
      switch (errorCode) {
        case CompileTimeErrorCode.CONST_EVAL_TYPE_BOOL:
        case CompileTimeErrorCode.CONST_EVAL_TYPE_BOOL_INT:
        case CompileTimeErrorCode.CONST_EVAL_TYPE_BOOL_NUM_STRING:
        case CompileTimeErrorCode.CONST_EVAL_TYPE_INT:
        case CompileTimeErrorCode.CONST_EVAL_TYPE_NUM:
        case CompileTimeErrorCode.CONST_EVAL_THROWS_EXCEPTION:
        case CompileTimeErrorCode.CONST_EVAL_THROWS_IDBZE:
        case CompileTimeErrorCode.CONST_WITH_NON_CONST:
        case CompileTimeErrorCode.CONST_WITH_NON_CONSTANT_ARGUMENT:
        case CompileTimeErrorCode.NON_CONSTANT_VALUE_IN_INITIALIZER:
        case CompileTimeErrorCode
            .CONST_CONSTRUCTOR_WITH_FIELD_INITIALIZED_BY_NON_CONST:
        case CompileTimeErrorCode.INVALID_CONSTANT:
        case CompileTimeErrorCode.MISSING_CONST_IN_LIST_LITERAL:
        case CompileTimeErrorCode.MISSING_CONST_IN_MAP_LITERAL:
        case CompileTimeErrorCode.MISSING_CONST_IN_SET_LITERAL:
          hasConstError = true;
      }
    }
  }
}

/**
 * A constructor declaration.
 *
 *    constructorDeclaration ::=
 *        constructorSignature [FunctionBody]?
 *      | constructorName formalParameterList ':' 'this' ('.' [SimpleIdentifier])? arguments
 *
 *    constructorSignature ::=
 *        'external'? constructorName formalParameterList initializerList?
 *      | 'external'? 'factory' factoryName formalParameterList initializerList?
 *      | 'external'? 'const'  constructorName formalParameterList initializerList?
 *
 *    constructorName ::=
 *        [SimpleIdentifier] ('.' [SimpleIdentifier])?
 *
 *    factoryName ::=
 *        [Identifier] ('.' [SimpleIdentifier])?
 *
 *    initializerList ::=
 *        ':' [ConstructorInitializer] (',' [ConstructorInitializer])*
 */
class ConstructorDeclarationImpl extends ClassMemberImpl
    implements ConstructorDeclaration {
  /**
   * The token for the 'external' keyword, or `null` if the constructor is not
   * external.
   */
  @override
  Token externalKeyword;

  /**
   * The token for the 'const' keyword, or `null` if the constructor is not a
   * const constructor.
   */
  @override
  Token constKeyword;

  /**
   * The token for the 'factory' keyword, or `null` if the constructor is not a
   * factory constructor.
   */
  @override
  Token factoryKeyword;

  /**
   * The type of object being created. This can be different than the type in
   * which the constructor is being declared if the constructor is the
   * implementation of a factory constructor.
   */
  IdentifierImpl _returnType;

  /**
   * The token for the period before the constructor name, or `null` if the
   * constructor being declared is unnamed.
   */
  @override
  Token period;

  /**
   * The name of the constructor, or `null` if the constructor being declared is
   * unnamed.
   */
  SimpleIdentifierImpl _name;

  /**
   * The parameters associated with the constructor.
   */
  FormalParameterListImpl _parameters;

  /**
   * The token for the separator (colon or equals) before the initializer list
   * or redirection, or `null` if there are no initializers.
   */
  @override
  Token separator;

  /**
   * The initializers associated with the constructor.
   */
  NodeList<ConstructorInitializer> _initializers;

  /**
   * The name of the constructor to which this constructor will be redirected,
   * or `null` if this is not a redirecting factory constructor.
   */
  ConstructorNameImpl _redirectedConstructor;

  /**
   * The body of the constructor, or `null` if the constructor does not have a
   * body.
   */
  FunctionBodyImpl _body;

  /**
   * The element associated with this constructor, or `null` if the AST
   * structure has not been resolved or if this constructor could not be
   * resolved.
   */
  @override
  ConstructorElement declaredElement;

  /**
   * Initialize a newly created constructor declaration. The [externalKeyword]
   * can be `null` if the constructor is not external. Either or both of the
   * [comment] and [metadata] can be `null` if the constructor does not have the
   * corresponding attribute. The [constKeyword] can be `null` if the
   * constructor cannot be used to create a constant. The [factoryKeyword] can
   * be `null` if the constructor is not a factory. The [period] and [name] can
   * both be `null` if the constructor is not a named constructor. The
   * [separator] can be `null` if the constructor does not have any initializers
   * and does not redirect to a different constructor. The list of
   * [initializers] can be `null` if the constructor does not have any
   * initializers. The [redirectedConstructor] can be `null` if the constructor
   * does not redirect to a different constructor. The [body] can be `null` if
   * the constructor does not have a body.
   */
  ConstructorDeclarationImpl(
      CommentImpl comment,
      List<Annotation> metadata,
      this.externalKeyword,
      this.constKeyword,
      this.factoryKeyword,
      IdentifierImpl returnType,
      this.period,
      SimpleIdentifierImpl name,
      FormalParameterListImpl parameters,
      this.separator,
      List<ConstructorInitializer> initializers,
      ConstructorNameImpl redirectedConstructor,
      FunctionBodyImpl body)
      : super(comment, metadata) {
    _returnType = _becomeParentOf(returnType);
    _name = _becomeParentOf(name);
    _parameters = _becomeParentOf(parameters);
    _initializers =
        new NodeListImpl<ConstructorInitializer>(this, initializers);
    _redirectedConstructor = _becomeParentOf(redirectedConstructor);
    _body = _becomeParentOf(body);
  }

  @override
  FunctionBody get body => _body;

  @override
  void set body(FunctionBody functionBody) {
    _body = _becomeParentOf(functionBody as FunctionBodyImpl);
  }

  @override
  Iterable<SyntacticEntity> get childEntities => super._childEntities
    ..add(externalKeyword)
    ..add(constKeyword)
    ..add(factoryKeyword)
    ..add(_returnType)
    ..add(period)
    ..add(_name)
    ..add(_parameters)
    ..add(separator)
    ..addAll(initializers)
    ..add(_redirectedConstructor)
    ..add(_body);

  @deprecated
  @override
  ConstructorElement get element => declaredElement;

  @deprecated
  @override
  set element(ConstructorElement element) {
    declaredElement = element;
  }

  @override
  Token get endToken {
    if (_body != null) {
      return _body.endToken;
    } else if (!_initializers.isEmpty) {
      return _initializers.endToken;
    }
    return _parameters.endToken;
  }

  @override
  Token get firstTokenAfterCommentAndMetadata {
    Token leftMost =
        Token.lexicallyFirst([externalKeyword, constKeyword, factoryKeyword]);
    if (leftMost != null) {
      return leftMost;
    }
    return _returnType.beginToken;
  }

  @override
  NodeList<ConstructorInitializer> get initializers => _initializers;

  @override
  SimpleIdentifier get name => _name;

  @override
  void set name(SimpleIdentifier identifier) {
    _name = _becomeParentOf(identifier as SimpleIdentifierImpl);
  }

  @override
  FormalParameterList get parameters => _parameters;

  @override
  void set parameters(FormalParameterList parameters) {
    _parameters = _becomeParentOf(parameters as FormalParameterListImpl);
  }

  @override
  ConstructorName get redirectedConstructor => _redirectedConstructor;

  @override
  void set redirectedConstructor(ConstructorName redirectedConstructor) {
    _redirectedConstructor =
        _becomeParentOf(redirectedConstructor as ConstructorNameImpl);
  }

  @override
  Identifier get returnType => _returnType;

  @override
  void set returnType(Identifier typeName) {
    _returnType = _becomeParentOf(typeName as IdentifierImpl);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) =>
      visitor.visitConstructorDeclaration(this);

  @override
  void visitChildren(AstVisitor visitor) {
    super.visitChildren(visitor);
    _returnType?.accept(visitor);
    _name?.accept(visitor);
    _parameters?.accept(visitor);
    _initializers.accept(visitor);
    _redirectedConstructor?.accept(visitor);
    _body?.accept(visitor);
  }
}

/**
 * The initialization of a field within a constructor's initialization list.
 *
 *    fieldInitializer ::=
 *        ('this' '.')? [SimpleIdentifier] '=' [Expression]
 */
class ConstructorFieldInitializerImpl extends ConstructorInitializerImpl
    implements ConstructorFieldInitializer {
  /**
   * The token for the 'this' keyword, or `null` if there is no 'this' keyword.
   */
  @override
  Token thisKeyword;

  /**
   * The token for the period after the 'this' keyword, or `null` if there is no
   * 'this' keyword.
   */
  @override
  Token period;

  /**
   * The name of the field being initialized.
   */
  SimpleIdentifierImpl _fieldName;

  /**
   * The token for the equal sign between the field name and the expression.
   */
  @override
  Token equals;

  /**
   * The expression computing the value to which the field will be initialized.
   */
  ExpressionImpl _expression;

  /**
   * Initialize a newly created field initializer to initialize the field with
   * the given name to the value of the given expression. The [thisKeyword] and
   * [period] can be `null` if the 'this' keyword was not specified.
   */
  ConstructorFieldInitializerImpl(this.thisKeyword, this.period,
      SimpleIdentifierImpl fieldName, this.equals, ExpressionImpl expression) {
    _fieldName = _becomeParentOf(fieldName);
    _expression = _becomeParentOf(expression);
  }

  @override
  Token get beginToken {
    if (thisKeyword != null) {
      return thisKeyword;
    }
    return _fieldName.beginToken;
  }

  @override
  Iterable<SyntacticEntity> get childEntities => new ChildEntities()
    ..add(thisKeyword)
    ..add(period)
    ..add(_fieldName)
    ..add(equals)
    ..add(_expression);

  @override
  Token get endToken => _expression.endToken;

  @override
  Expression get expression => _expression;

  @override
  void set expression(Expression expression) {
    _expression = _becomeParentOf(expression as ExpressionImpl);
  }

  @override
  SimpleIdentifier get fieldName => _fieldName;

  @override
  void set fieldName(SimpleIdentifier identifier) {
    _fieldName = _becomeParentOf(identifier as SimpleIdentifierImpl);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) =>
      visitor.visitConstructorFieldInitializer(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _fieldName?.accept(visitor);
    _expression?.accept(visitor);
  }
}

/**
 * A node that can occur in the initializer list of a constructor declaration.
 *
 *    constructorInitializer ::=
 *        [SuperConstructorInvocation]
 *      | [ConstructorFieldInitializer]
 *      | [RedirectingConstructorInvocation]
 */
abstract class ConstructorInitializerImpl extends AstNodeImpl
    implements ConstructorInitializer {}

/**
 * The name of the constructor.
 *
 *    constructorName ::=
 *        type ('.' identifier)?
 */
class ConstructorNameImpl extends AstNodeImpl implements ConstructorName {
  /**
   * The name of the type defining the constructor.
   */
  TypeNameImpl _type;

  /**
   * The token for the period before the constructor name, or `null` if the
   * specified constructor is the unnamed constructor.
   */
  @override
  Token period;

  /**
   * The name of the constructor, or `null` if the specified constructor is the
   * unnamed constructor.
   */
  SimpleIdentifierImpl _name;

  /**
   * The element associated with this constructor name based on static type
   * information, or `null` if the AST structure has not been resolved or if
   * this constructor name could not be resolved.
   */
  @override
  ConstructorElement staticElement;

  /**
   * Initialize a newly created constructor name. The [period] and [name] can be
   * `null` if the constructor being named is the unnamed constructor.
   */
  ConstructorNameImpl(
      TypeNameImpl type, this.period, SimpleIdentifierImpl name) {
    _type = _becomeParentOf(type);
    _name = _becomeParentOf(name);
  }

  @override
  Token get beginToken => _type.beginToken;

  @override
  Iterable<SyntacticEntity> get childEntities =>
      new ChildEntities()..add(_type)..add(period)..add(_name);

  @override
  Token get endToken {
    if (_name != null) {
      return _name.endToken;
    }
    return _type.endToken;
  }

  @override
  SimpleIdentifier get name => _name;

  @override
  void set name(SimpleIdentifier name) {
    _name = _becomeParentOf(name as SimpleIdentifierImpl);
  }

  @override
  TypeName get type => _type;

  @override
  void set type(TypeName type) {
    _type = _becomeParentOf(type as TypeNameImpl);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitConstructorName(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _type?.accept(visitor);
    _name?.accept(visitor);
  }
}

/**
 * A continue statement.
 *
 *    continueStatement ::=
 *        'continue' [SimpleIdentifier]? ';'
 */
class ContinueStatementImpl extends StatementImpl implements ContinueStatement {
  /**
   * The token representing the 'continue' keyword.
   */
  @override
  Token continueKeyword;

  /**
   * The label associated with the statement, or `null` if there is no label.
   */
  SimpleIdentifierImpl _label;

  /**
   * The semicolon terminating the statement.
   */
  @override
  Token semicolon;

  /**
   * The AstNode which this continue statement is continuing to.  This will be
   * either a Statement (in the case of continuing a loop) or a SwitchMember
   * (in the case of continuing from one switch case to another).  Null if the
   * AST has not yet been resolved or if the target could not be resolved.
   * Note that if the source code has errors, the target may be invalid (e.g.
   * the target may be in an enclosing function).
   */
  AstNode target;

  /**
   * Initialize a newly created continue statement. The [label] can be `null` if
   * there is no label associated with the statement.
   */
  ContinueStatementImpl(
      this.continueKeyword, SimpleIdentifierImpl label, this.semicolon) {
    _label = _becomeParentOf(label);
  }

  @override
  Token get beginToken => continueKeyword;

  @override
  Iterable<SyntacticEntity> get childEntities =>
      new ChildEntities()..add(continueKeyword)..add(_label)..add(semicolon);

  @override
  Token get endToken => semicolon;

  @override
  SimpleIdentifier get label => _label;

  @override
  void set label(SimpleIdentifier identifier) {
    _label = _becomeParentOf(identifier as SimpleIdentifierImpl);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitContinueStatement(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _label?.accept(visitor);
  }
}

/**
 * A node that represents the declaration of one or more names. Each declared
 * name is visible within a name scope.
 */
abstract class DeclarationImpl extends AnnotatedNodeImpl
    implements Declaration {
  /**
   * Initialize a newly created declaration. Either or both of the [comment] and
   * [metadata] can be `null` if the declaration does not have the corresponding
   * attribute.
   */
  DeclarationImpl(CommentImpl comment, List<Annotation> metadata)
      : super(comment, metadata);
}

/**
 * The declaration of a single identifier.
 *
 *    declaredIdentifier ::=
 *        [Annotation] finalConstVarOrType [SimpleIdentifier]
 */
class DeclaredIdentifierImpl extends DeclarationImpl
    implements DeclaredIdentifier {
  /**
   * The token representing either the 'final', 'const' or 'var' keyword, or
   * `null` if no keyword was used.
   */
  @override
  Token keyword;

  /**
   * The name of the declared type of the parameter, or `null` if the parameter
   * does not have a declared type.
   */
  TypeAnnotationImpl _type;

  /**
   * The name of the variable being declared.
   */
  SimpleIdentifierImpl _identifier;

  /**
   * Initialize a newly created formal parameter. Either or both of the
   * [comment] and [metadata] can be `null` if the declaration does not have the
   * corresponding attribute. The [keyword] can be `null` if a type name is
   * given. The [type] must be `null` if the keyword is 'var'.
   */
  DeclaredIdentifierImpl(CommentImpl comment, List<Annotation> metadata,
      this.keyword, TypeAnnotationImpl type, SimpleIdentifierImpl identifier)
      : super(comment, metadata) {
    _type = _becomeParentOf(type);
    _identifier = _becomeParentOf(identifier);
  }

  @override
  Iterable<SyntacticEntity> get childEntities =>
      super._childEntities..add(keyword)..add(_type)..add(_identifier);

  @override
  LocalVariableElement get declaredElement {
    if (_identifier == null) {
      return null;
    }
    return _identifier.staticElement as LocalVariableElement;
  }

  @override
  LocalVariableElement get element => declaredElement;

  @override
  Token get endToken => _identifier.endToken;

  @override
  Token get firstTokenAfterCommentAndMetadata {
    if (keyword != null) {
      return keyword;
    } else if (_type != null) {
      return _type.beginToken;
    }
    return _identifier.beginToken;
  }

  @override
  SimpleIdentifier get identifier => _identifier;

  @override
  void set identifier(SimpleIdentifier identifier) {
    _identifier = _becomeParentOf(identifier as SimpleIdentifierImpl);
  }

  @override
  bool get isConst => keyword?.keyword == Keyword.CONST;

  @override
  bool get isFinal => keyword?.keyword == Keyword.FINAL;

  @override
  TypeAnnotation get type => _type;

  @override
  void set type(TypeAnnotation type) {
    _type = _becomeParentOf(type as TypeAnnotationImpl);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitDeclaredIdentifier(this);

  @override
  void visitChildren(AstVisitor visitor) {
    super.visitChildren(visitor);
    _type?.accept(visitor);
    _identifier?.accept(visitor);
  }
}

/**
 * A simple identifier that declares a name.
 */
// TODO(rnystrom): Consider making this distinct from [SimpleIdentifier] and
// get rid of all of the:
//
//     if (node.inDeclarationContext()) { ... }
//
// code and instead visit this separately. A declaration is semantically pretty
// different from a use, so using the same node type doesn't seem to buy us
// much.
class DeclaredSimpleIdentifier extends SimpleIdentifierImpl {
  DeclaredSimpleIdentifier(Token token) : super(token);

  @override
  bool inDeclarationContext() => true;
}

/**
 * A formal parameter with a default value. There are two kinds of parameters
 * that are both represented by this class: named formal parameters and
 * positional formal parameters.
 *
 *    defaultFormalParameter ::=
 *        [NormalFormalParameter] ('=' [Expression])?
 *
 *    defaultNamedParameter ::=
 *        [NormalFormalParameter] (':' [Expression])?
 */
class DefaultFormalParameterImpl extends FormalParameterImpl
    implements DefaultFormalParameter {
  /**
   * The formal parameter with which the default value is associated.
   */
  NormalFormalParameterImpl _parameter;

  /**
   * The kind of this parameter.
   */
  @override
  ParameterKind kind;

  /**
   * The token separating the parameter from the default value, or `null` if
   * there is no default value.
   */
  @override
  Token separator;

  /**
   * The expression computing the default value for the parameter, or `null` if
   * there is no default value.
   */
  ExpressionImpl _defaultValue;

  /**
   * Initialize a newly created default formal parameter. The [separator] and
   * [defaultValue] can be `null` if there is no default value.
   */
  DefaultFormalParameterImpl(NormalFormalParameterImpl parameter, this.kind,
      this.separator, ExpressionImpl defaultValue) {
    _parameter = _becomeParentOf(parameter);
    _defaultValue = _becomeParentOf(defaultValue);
  }

  @override
  Token get beginToken => _parameter.beginToken;

  @override
  Iterable<SyntacticEntity> get childEntities =>
      new ChildEntities()..add(_parameter)..add(separator)..add(_defaultValue);

  @override
  Token get covariantKeyword => null;

  @override
  Expression get defaultValue => _defaultValue;

  @override
  void set defaultValue(Expression expression) {
    _defaultValue = _becomeParentOf(expression as ExpressionImpl);
  }

  @override
  Token get endToken {
    if (_defaultValue != null) {
      return _defaultValue.endToken;
    }
    return _parameter.endToken;
  }

  @override
  SimpleIdentifier get identifier => _parameter.identifier;

  @override
  bool get isConst => _parameter != null && _parameter.isConst;

  @override
  bool get isFinal => _parameter != null && _parameter.isFinal;

  @override
  NodeList<Annotation> get metadata => _parameter.metadata;

  @override
  NormalFormalParameter get parameter => _parameter;

  @override
  void set parameter(NormalFormalParameter formalParameter) {
    _parameter = _becomeParentOf(formalParameter as NormalFormalParameterImpl);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) =>
      visitor.visitDefaultFormalParameter(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _parameter?.accept(visitor);
    _defaultValue?.accept(visitor);
  }
}

/**
 * A node that represents a directive.
 *
 *    directive ::=
 *        [ExportDirective]
 *      | [ImportDirective]
 *      | [LibraryDirective]
 *      | [PartDirective]
 *      | [PartOfDirective]
 */
abstract class DirectiveImpl extends AnnotatedNodeImpl implements Directive {
  /**
   * The element associated with this directive, or `null` if the AST structure
   * has not been resolved or if this directive could not be resolved.
   */
  Element _element;

  /**
   * Initialize a newly create directive. Either or both of the [comment] and
   * [metadata] can be `null` if the directive does not have the corresponding
   * attribute.
   */
  DirectiveImpl(CommentImpl comment, List<Annotation> metadata)
      : super(comment, metadata);

  @override
  Element get element => _element;

  /**
   * Set the element associated with this directive to be the given [element].
   */
  void set element(Element element) {
    _element = element;
  }
}

/**
 * A do statement.
 *
 *    doStatement ::=
 *        'do' [Statement] 'while' '(' [Expression] ')' ';'
 */
class DoStatementImpl extends StatementImpl implements DoStatement {
  /**
   * The token representing the 'do' keyword.
   */
  @override
  Token doKeyword;

  /**
   * The body of the loop.
   */
  StatementImpl _body;

  /**
   * The token representing the 'while' keyword.
   */
  @override
  Token whileKeyword;

  /**
   * The left parenthesis.
   */
  Token leftParenthesis;

  /**
   * The condition that determines when the loop will terminate.
   */
  ExpressionImpl _condition;

  /**
   * The right parenthesis.
   */
  @override
  Token rightParenthesis;

  /**
   * The semicolon terminating the statement.
   */
  @override
  Token semicolon;

  /**
   * Initialize a newly created do loop.
   */
  DoStatementImpl(
      this.doKeyword,
      StatementImpl body,
      this.whileKeyword,
      this.leftParenthesis,
      ExpressionImpl condition,
      this.rightParenthesis,
      this.semicolon) {
    _body = _becomeParentOf(body);
    _condition = _becomeParentOf(condition);
  }

  @override
  Token get beginToken => doKeyword;

  @override
  Statement get body => _body;

  @override
  void set body(Statement statement) {
    _body = _becomeParentOf(statement as StatementImpl);
  }

  @override
  Iterable<SyntacticEntity> get childEntities => new ChildEntities()
    ..add(doKeyword)
    ..add(_body)
    ..add(whileKeyword)
    ..add(leftParenthesis)
    ..add(_condition)
    ..add(rightParenthesis)
    ..add(semicolon);

  @override
  Expression get condition => _condition;

  @override
  void set condition(Expression expression) {
    _condition = _becomeParentOf(expression as ExpressionImpl);
  }

  @override
  Token get endToken => semicolon;

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitDoStatement(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _body?.accept(visitor);
    _condition?.accept(visitor);
  }
}

/**
 * A dotted name, used in a configuration within an import or export directive.
 *
 *    dottedName ::=
 *        [SimpleIdentifier] ('.' [SimpleIdentifier])*
 */
class DottedNameImpl extends AstNodeImpl implements DottedName {
  /**
   * The components of the identifier.
   */
  NodeList<SimpleIdentifier> _components;

  /**
   * Initialize a newly created dotted name.
   */
  DottedNameImpl(List<SimpleIdentifier> components) {
    _components = new NodeListImpl<SimpleIdentifier>(this, components);
  }

  @override
  Token get beginToken => _components.beginToken;

  @override
  // TODO(paulberry): add "." tokens.
  Iterable<SyntacticEntity> get childEntities =>
      new ChildEntities()..addAll(_components);

  @override
  NodeList<SimpleIdentifier> get components => _components;

  @override
  Token get endToken => _components.endToken;

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitDottedName(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _components.accept(visitor);
  }
}

/**
 * A floating point literal expression.
 *
 *    doubleLiteral ::=
 *        decimalDigit+ ('.' decimalDigit*)? exponent?
 *      | '.' decimalDigit+ exponent?
 *
 *    exponent ::=
 *        ('e' | 'E') ('+' | '-')? decimalDigit+
 */
class DoubleLiteralImpl extends LiteralImpl implements DoubleLiteral {
  /**
   * The token representing the literal.
   */
  @override
  Token literal;

  /**
   * The value of the literal.
   */
  @override
  double value;

  /**
   * Initialize a newly created floating point literal.
   */
  DoubleLiteralImpl(this.literal, this.value);

  @override
  Token get beginToken => literal;

  @override
  Iterable<SyntacticEntity> get childEntities =>
      new ChildEntities()..add(literal);

  @override
  Token get endToken => literal;

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitDoubleLiteral(this);

  @override
  void visitChildren(AstVisitor visitor) {
    // There are no children to visit.
  }
}

/**
 * An empty function body, which can only appear in constructors or abstract
 * methods.
 *
 *    emptyFunctionBody ::=
 *        ';'
 */
class EmptyFunctionBodyImpl extends FunctionBodyImpl
    implements EmptyFunctionBody {
  /**
   * The token representing the semicolon that marks the end of the function
   * body.
   */
  @override
  Token semicolon;

  /**
   * Initialize a newly created function body.
   */
  EmptyFunctionBodyImpl(this.semicolon);

  @override
  Token get beginToken => semicolon;

  @override
  Iterable<SyntacticEntity> get childEntities =>
      new ChildEntities()..add(semicolon);

  @override
  Token get endToken => semicolon;

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitEmptyFunctionBody(this);

  @override
  void visitChildren(AstVisitor visitor) {
    // Empty function bodies have no children.
  }
}

/**
 * An empty statement.
 *
 *    emptyStatement ::=
 *        ';'
 */
class EmptyStatementImpl extends StatementImpl implements EmptyStatement {
  /**
   * The semicolon terminating the statement.
   */
  Token semicolon;

  /**
   * Initialize a newly created empty statement.
   */
  EmptyStatementImpl(this.semicolon);

  @override
  Token get beginToken => semicolon;

  @override
  Iterable<SyntacticEntity> get childEntities =>
      new ChildEntities()..add(semicolon);

  @override
  Token get endToken => semicolon;

  @override
  bool get isSynthetic => semicolon.isSynthetic;

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitEmptyStatement(this);

  @override
  void visitChildren(AstVisitor visitor) {
    // There are no children to visit.
  }
}

/**
 * The declaration of an enum constant.
 */
class EnumConstantDeclarationImpl extends DeclarationImpl
    implements EnumConstantDeclaration {
  /**
   * The name of the constant.
   */
  SimpleIdentifierImpl _name;

  /**
   * Initialize a newly created enum constant declaration. Either or both of the
   * [comment] and [metadata] can be `null` if the constant does not have the
   * corresponding attribute. (Technically, enum constants cannot have metadata,
   * but we allow it for consistency.)
   */
  EnumConstantDeclarationImpl(
      CommentImpl comment, List<Annotation> metadata, SimpleIdentifierImpl name)
      : super(comment, metadata) {
    _name = _becomeParentOf(name);
  }

  @override
  Iterable<SyntacticEntity> get childEntities =>
      super._childEntities..add(_name);

  @override
  FieldElement get declaredElement => _name?.staticElement as FieldElement;

  @deprecated
  @override
  FieldElement get element => declaredElement;

  @override
  Token get endToken => _name.endToken;

  @override
  Token get firstTokenAfterCommentAndMetadata => _name.beginToken;

  @override
  SimpleIdentifier get name => _name;

  @override
  void set name(SimpleIdentifier name) {
    _name = _becomeParentOf(name as SimpleIdentifierImpl);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) =>
      visitor.visitEnumConstantDeclaration(this);

  @override
  void visitChildren(AstVisitor visitor) {
    super.visitChildren(visitor);
    _name?.accept(visitor);
  }
}

/**
 * The declaration of an enumeration.
 *
 *    enumType ::=
 *        metadata 'enum' [SimpleIdentifier] '{' [SimpleIdentifier] (',' [SimpleIdentifier])* (',')? '}'
 */
class EnumDeclarationImpl extends NamedCompilationUnitMemberImpl
    implements EnumDeclaration {
  /**
   * The 'enum' keyword.
   */
  @override
  Token enumKeyword;

  /**
   * The left curly bracket.
   */
  @override
  Token leftBracket;

  /**
   * The enumeration constants being declared.
   */
  NodeList<EnumConstantDeclaration> _constants;

  /**
   * The right curly bracket.
   */
  @override
  Token rightBracket;

  /**
   * Initialize a newly created enumeration declaration. Either or both of the
   * [comment] and [metadata] can be `null` if the declaration does not have the
   * corresponding attribute. The list of [constants] must contain at least one
   * value.
   */
  EnumDeclarationImpl(
      CommentImpl comment,
      List<Annotation> metadata,
      this.enumKeyword,
      SimpleIdentifierImpl name,
      this.leftBracket,
      List<EnumConstantDeclaration> constants,
      this.rightBracket)
      : super(comment, metadata, name) {
    _constants = new NodeListImpl<EnumConstantDeclaration>(this, constants);
  }

  @override
  // TODO(brianwilkerson) Add commas?
  Iterable<SyntacticEntity> get childEntities => super._childEntities
    ..add(enumKeyword)
    ..add(_name)
    ..add(leftBracket)
    ..addAll(_constants)
    ..add(rightBracket);

  @override
  NodeList<EnumConstantDeclaration> get constants => _constants;

  @override
  ClassElement get declaredElement => _name?.staticElement as ClassElement;

  @deprecated
  @override
  ClassElement get element => declaredElement;

  @override
  Token get endToken => rightBracket;

  @override
  Token get firstTokenAfterCommentAndMetadata => enumKeyword;

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitEnumDeclaration(this);

  @override
  void visitChildren(AstVisitor visitor) {
    super.visitChildren(visitor);
    _name?.accept(visitor);
    _constants.accept(visitor);
  }
}

/**
 * Ephemeral identifiers are created as needed to mimic the presence of an empty
 * identifier.
 */
class EphemeralIdentifier extends SimpleIdentifierImpl {
  EphemeralIdentifier(AstNode parent, int location)
      : super(new StringToken(TokenType.IDENTIFIER, "", location)) {
    (parent as AstNodeImpl)._becomeParentOf(this);
  }
}

/**
 * An export directive.
 *
 *    exportDirective ::=
 *        [Annotation] 'export' [StringLiteral] [Combinator]* ';'
 */
class ExportDirectiveImpl extends NamespaceDirectiveImpl
    implements ExportDirective {
  /**
   * Initialize a newly created export directive. Either or both of the
   * [comment] and [metadata] can be `null` if the directive does not have the
   * corresponding attribute. The list of [combinators] can be `null` if there
   * are no combinators.
   */
  ExportDirectiveImpl(
      CommentImpl comment,
      List<Annotation> metadata,
      Token keyword,
      StringLiteralImpl libraryUri,
      List<Configuration> configurations,
      List<Combinator> combinators,
      Token semicolon)
      : super(comment, metadata, keyword, libraryUri, configurations,
            combinators, semicolon);

  @override
  Iterable<SyntacticEntity> get childEntities => super._childEntities
    ..add(_uri)
    ..addAll(combinators)
    ..add(semicolon);

  @override
  ExportElement get element => super.element as ExportElement;

  @override
  LibraryElement get uriElement {
    if (element != null) {
      return element.exportedLibrary;
    }
    return null;
  }

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitExportDirective(this);

  @override
  void visitChildren(AstVisitor visitor) {
    super.visitChildren(visitor);
    combinators.accept(visitor);
  }
}

/**
 * A function body consisting of a single expression.
 *
 *    expressionFunctionBody ::=
 *        'async'? '=>' [Expression] ';'
 */
class ExpressionFunctionBodyImpl extends FunctionBodyImpl
    implements ExpressionFunctionBody {
  /**
   * The token representing the 'async' keyword, or `null` if there is no such
   * keyword.
   */
  @override
  Token keyword;

  /**
   * The token introducing the expression that represents the body of the
   * function.
   */
  @override
  Token functionDefinition;

  /**
   * The expression representing the body of the function.
   */
  ExpressionImpl _expression;

  /**
   * The semicolon terminating the statement.
   */
  @override
  Token semicolon;

  /**
   * Initialize a newly created function body consisting of a block of
   * statements. The [keyword] can be `null` if the function body is not an
   * async function body.
   */
  ExpressionFunctionBodyImpl(this.keyword, this.functionDefinition,
      ExpressionImpl expression, this.semicolon) {
    _expression = _becomeParentOf(expression);
  }

  @override
  Token get beginToken {
    if (keyword != null) {
      return keyword;
    }
    return functionDefinition;
  }

  @override
  Iterable<SyntacticEntity> get childEntities => new ChildEntities()
    ..add(keyword)
    ..add(functionDefinition)
    ..add(_expression)
    ..add(semicolon);

  @override
  Token get endToken {
    if (semicolon != null) {
      return semicolon;
    }
    return _expression.endToken;
  }

  @override
  Expression get expression => _expression;

  @override
  void set expression(Expression expression) {
    _expression = _becomeParentOf(expression as ExpressionImpl);
  }

  @override
  bool get isAsynchronous => keyword != null;

  @override
  bool get isSynchronous => keyword == null;

  @override
  E accept<E>(AstVisitor<E> visitor) =>
      visitor.visitExpressionFunctionBody(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _expression?.accept(visitor);
  }
}

/**
 * A node that represents an expression.
 *
 *    expression ::=
 *        [AssignmentExpression]
 *      | [ConditionalExpression] cascadeSection*
 *      | [ThrowExpression]
 */
abstract class ExpressionImpl extends AstNodeImpl
    implements CollectionElementImpl, Expression {
  /**
   * The static type of this expression, or `null` if the AST structure has not
   * been resolved.
   */
  @override
  DartType staticType;

  /**
   * Return the best parameter element information available for this
   * expression. If type propagation was able to find a better parameter element
   * than static analysis, that type will be returned. Otherwise, the result of
   * static analysis will be returned.
   */
  @override
  @deprecated
  ParameterElement get bestParameterElement => staticParameterElement;

  @override
  @deprecated
  DartType get bestType => staticType ?? DynamicTypeImpl.instance;

  /**
   * An expression _e_ is said to _occur in a constant context_,
   * * if _e_ is an element of a constant list literal, or a key or value of an
   *   entry of a constant map literal.
   * * if _e_ is an actual argument of a constant object expression or of a
   *   metadata annotation.
   * * if _e_ is the initializing expression of a constant variable declaration.
   * * if _e_ is a switch case expression.
   * * if _e_ is an immediate subexpression of an expression _e1_ which occurs
   *   in a constant context, unless _e1_ is a `throw` expression or a function
   *   literal.
   *
   * This roughly means that everything which is inside a syntactically constant
   * expression is in a constant context. A `throw` expression is currently not
   * allowed in a constant expression, but extensions affecting that status may
   * be considered. A similar situation arises for function literals.
   *
   * Note that the default value of an optional formal parameter is _not_ a
   * constant context. This choice reserves some freedom to modify the semantics
   * of default values.
   */
  bool get inConstantContext {
    AstNode child = this;
    while (child is Expression ||
        child is ArgumentList ||
        child is MapLiteralEntry ||
        child is SpreadElement ||
        child is IfElement ||
        child is ForElement) {
      AstNode parent = child.parent;
      if (parent is TypedLiteralImpl && parent.constKeyword != null) {
        // Inside an explicitly `const` list or map literal.
        return true;
      } else if (parent is InstanceCreationExpression &&
          parent.keyword?.keyword == Keyword.CONST) {
        // Inside an explicitly `const` instance creation expression.
        return true;
      } else if (parent is Annotation) {
        // Inside an annotation.
        return true;
      } else if (parent is VariableDeclaration) {
        AstNode grandParent = parent.parent;
        // Inside the initializer for a `const` variable declaration.
        return grandParent is VariableDeclarationList &&
            grandParent.keyword?.keyword == Keyword.CONST;
      } else if (parent is SwitchCase) {
        // Inside a switch case.
        return true;
      }
      child = parent;
    }
    return false;
  }

  @override
  bool get isAssignable => false;

  @deprecated
  @override
  ParameterElement get propagatedParameterElement => null;

  @deprecated
  @override
  DartType get propagatedType => null;

  @deprecated
  @override
  set propagatedType(DartType type) {}

  @override
  ParameterElement get staticParameterElement {
    AstNode parent = this.parent;
    if (parent is ArgumentListImpl) {
      return parent._getStaticParameterElementFor(this);
    } else if (parent is IndexExpressionImpl) {
      if (identical(parent.index, this)) {
        return parent._staticParameterElementForIndex;
      }
    } else if (parent is BinaryExpressionImpl) {
      if (identical(parent.rightOperand, this)) {
        var parameters = parent.staticInvokeType?.parameters;
        if (parameters != null && parameters.isNotEmpty) {
          return parameters[0];
        }
        return null;
      }
    } else if (parent is AssignmentExpressionImpl) {
      if (identical(parent.rightHandSide, this)) {
        return parent._staticParameterElementForRightHandSide;
      }
    } else if (parent is PrefixExpressionImpl) {
      return parent._staticParameterElementForOperand;
    } else if (parent is PostfixExpressionImpl) {
      return parent._staticParameterElementForOperand;
    }
    return null;
  }

  @override
  Expression get unParenthesized => this;
}

/**
 * An expression used as a statement.
 *
 *    expressionStatement ::=
 *        [Expression]? ';'
 */
class ExpressionStatementImpl extends StatementImpl
    implements ExpressionStatement {
  /**
   * The expression that comprises the statement.
   */
  ExpressionImpl _expression;

  /**
   * The semicolon terminating the statement, or `null` if the expression is a
   * function expression and therefore isn't followed by a semicolon.
   */
  @override
  Token semicolon;

  /**
   * Initialize a newly created expression statement.
   */
  ExpressionStatementImpl(ExpressionImpl expression, this.semicolon) {
    _expression = _becomeParentOf(expression);
  }

  @override
  Token get beginToken => _expression.beginToken;

  @override
  Iterable<SyntacticEntity> get childEntities =>
      new ChildEntities()..add(_expression)..add(semicolon);

  @override
  Token get endToken {
    if (semicolon != null) {
      return semicolon;
    }
    return _expression.endToken;
  }

  @override
  Expression get expression => _expression;

  @override
  void set expression(Expression expression) {
    _expression = _becomeParentOf(expression as ExpressionImpl);
  }

  @override
  bool get isSynthetic => _expression.isSynthetic && semicolon.isSynthetic;

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitExpressionStatement(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _expression?.accept(visitor);
  }
}

/**
 * The "extends" clause in a class declaration.
 *
 *    extendsClause ::=
 *        'extends' [TypeName]
 */
class ExtendsClauseImpl extends AstNodeImpl implements ExtendsClause {
  /**
   * The token representing the 'extends' keyword.
   */
  @override
  Token extendsKeyword;

  /**
   * The name of the class that is being extended.
   */
  TypeNameImpl _superclass;

  /**
   * Initialize a newly created extends clause.
   */
  ExtendsClauseImpl(this.extendsKeyword, TypeNameImpl superclass) {
    _superclass = _becomeParentOf(superclass);
  }

  @override
  Token get beginToken => extendsKeyword;

  @override
  Iterable<SyntacticEntity> get childEntities =>
      new ChildEntities()..add(extendsKeyword)..add(_superclass);

  @override
  Token get endToken => _superclass.endToken;

  @override
  TypeName get superclass => _superclass;

  @override
  void set superclass(TypeName name) {
    _superclass = _becomeParentOf(name as TypeNameImpl);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitExtendsClause(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _superclass?.accept(visitor);
  }
}

/**
 * The declaration of one or more fields of the same type.
 *
 *    fieldDeclaration ::=
 *        'static'? [VariableDeclarationList] ';'
 */
class FieldDeclarationImpl extends ClassMemberImpl implements FieldDeclaration {
  /**
   * The 'covariant' keyword, or `null` if the keyword was not used.
   */
  @override
  Token covariantKeyword;

  /**
   * The token representing the 'static' keyword, or `null` if the fields are
   * not static.
   */
  @override
  Token staticKeyword;

  /**
   * The fields being declared.
   */
  VariableDeclarationListImpl _fieldList;

  /**
   * The semicolon terminating the declaration.
   */
  @override
  Token semicolon;

  /**
   * Initialize a newly created field declaration. Either or both of the
   * [comment] and [metadata] can be `null` if the declaration does not have the
   * corresponding attribute. The [staticKeyword] can be `null` if the field is
   * not a static field.
   */
  FieldDeclarationImpl(
      CommentImpl comment,
      List<Annotation> metadata,
      this.covariantKeyword,
      this.staticKeyword,
      VariableDeclarationListImpl fieldList,
      this.semicolon)
      : super(comment, metadata) {
    _fieldList = _becomeParentOf(fieldList);
  }

  @override
  Iterable<SyntacticEntity> get childEntities =>
      super._childEntities..add(staticKeyword)..add(_fieldList)..add(semicolon);

  @override
  Element get declaredElement => null;

  @deprecated
  @override
  Element get element => null;

  @override
  Token get endToken => semicolon;

  @override
  VariableDeclarationList get fields => _fieldList;

  @override
  void set fields(VariableDeclarationList fields) {
    _fieldList = _becomeParentOf(fields as VariableDeclarationListImpl);
  }

  @override
  Token get firstTokenAfterCommentAndMetadata {
    if (covariantKeyword != null) {
      return covariantKeyword;
    } else if (staticKeyword != null) {
      return staticKeyword;
    }
    return _fieldList.beginToken;
  }

  @override
  bool get isStatic => staticKeyword != null;

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitFieldDeclaration(this);

  @override
  void visitChildren(AstVisitor visitor) {
    super.visitChildren(visitor);
    _fieldList?.accept(visitor);
  }
}

/**
 * A field formal parameter.
 *
 *    fieldFormalParameter ::=
 *        ('final' [TypeName] | 'const' [TypeName] | 'var' | [TypeName])?
 *        'this' '.' [SimpleIdentifier] ([TypeParameterList]? [FormalParameterList])?
 */
class FieldFormalParameterImpl extends NormalFormalParameterImpl
    implements FieldFormalParameter {
  /**
   * The token representing either the 'final', 'const' or 'var' keyword, or
   * `null` if no keyword was used.
   */
  @override
  Token keyword;

  /**
   * The name of the declared type of the parameter, or `null` if the parameter
   * does not have a declared type.
   */
  TypeAnnotationImpl _type;

  /**
   * The token representing the 'this' keyword.
   */
  @override
  Token thisKeyword;

  /**
   * The token representing the period.
   */
  @override
  Token period;

  /**
   * The type parameters associated with the method, or `null` if the method is
   * not a generic method.
   */
  TypeParameterListImpl _typeParameters;

  /**
   * The parameters of the function-typed parameter, or `null` if this is not a
   * function-typed field formal parameter.
   */
  FormalParameterListImpl _parameters;

  /**
   * Initialize a newly created formal parameter. Either or both of the
   * [comment] and [metadata] can be `null` if the parameter does not have the
   * corresponding attribute. The [keyword] can be `null` if there is a type.
   * The [type] must be `null` if the keyword is 'var'. The [thisKeyword] and
   * [period] can be `null` if the keyword 'this' was not provided.  The
   * [parameters] can be `null` if this is not a function-typed field formal
   * parameter.
   */
  FieldFormalParameterImpl(
      CommentImpl comment,
      List<Annotation> metadata,
      Token covariantKeyword,
      this.keyword,
      TypeAnnotationImpl type,
      this.thisKeyword,
      this.period,
      SimpleIdentifierImpl identifier,
      TypeParameterListImpl typeParameters,
      FormalParameterListImpl parameters)
      : super(comment, metadata, covariantKeyword, identifier) {
    _type = _becomeParentOf(type);
    _typeParameters = _becomeParentOf(typeParameters);
    _parameters = _becomeParentOf(parameters);
  }

  @override
  Token get beginToken {
    NodeList<Annotation> metadata = this.metadata;
    if (!metadata.isEmpty) {
      return metadata.beginToken;
    } else if (covariantKeyword != null) {
      return covariantKeyword;
    } else if (keyword != null) {
      return keyword;
    } else if (_type != null) {
      return _type.beginToken;
    }
    return thisKeyword;
  }

  @override
  Iterable<SyntacticEntity> get childEntities => super._childEntities
    ..add(keyword)
    ..add(_type)
    ..add(thisKeyword)
    ..add(period)
    ..add(identifier)
    ..add(_parameters);

  @override
  Token get endToken {
    if (_parameters != null) {
      return _parameters.endToken;
    }
    return identifier.endToken;
  }

  @override
  bool get isConst => keyword?.keyword == Keyword.CONST;

  @override
  bool get isFinal => keyword?.keyword == Keyword.FINAL;

  @override
  FormalParameterList get parameters => _parameters;

  @override
  void set parameters(FormalParameterList parameters) {
    _parameters = _becomeParentOf(parameters as FormalParameterListImpl);
  }

  @override
  TypeAnnotation get type => _type;

  @override
  void set type(TypeAnnotation type) {
    _type = _becomeParentOf(type as TypeAnnotationImpl);
  }

  @override
  TypeParameterList get typeParameters => _typeParameters;

  @override
  void set typeParameters(TypeParameterList typeParameters) {
    _typeParameters = _becomeParentOf(typeParameters as TypeParameterListImpl);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitFieldFormalParameter(this);

  @override
  void visitChildren(AstVisitor visitor) {
    super.visitChildren(visitor);
    _type?.accept(visitor);
    identifier?.accept(visitor);
    _typeParameters?.accept(visitor);
    _parameters?.accept(visitor);
  }
}

abstract class ForEachPartsImpl extends ForLoopPartsImpl
    implements ForEachParts {
  @override
  Token inKeyword;

  /**
   * The expression evaluated to produce the iterator.
   */
  ExpressionImpl _iterable;

  /**
   * Initialize a newly created for-each statement whose loop control variable
   * is declared internally (in the for-loop part). The [awaitKeyword] can be
   * `null` if this is not an asynchronous for loop.
   */
  ForEachPartsImpl(this.inKeyword, ExpressionImpl iterator) {
    _iterable = _becomeParentOf(iterator);
  }

  @override
  Token get beginToken => inKeyword;

  @override
  Iterable<SyntacticEntity> get childEntities =>
      new ChildEntities()..add(inKeyword)..add(_iterable);

  @override
  Token get endToken => _iterable.endToken;

  @override
  Expression get iterable => _iterable;

  void set iterable(Expression expression) {
    _iterable = _becomeParentOf(expression as ExpressionImpl);
  }

  @override
  void visitChildren(AstVisitor visitor) {
    _iterable?.accept(visitor);
  }
}

class ForEachPartsWithDeclarationImpl extends ForEachPartsImpl
    implements ForEachPartsWithDeclaration {
  /**
   * The declaration of the loop variable.
   */
  DeclaredIdentifierImpl _loopVariable;

  /**
   * Initialize a newly created for-each statement whose loop control variable
   * is declared internally (inside the for-loop part).
   */
  ForEachPartsWithDeclarationImpl(DeclaredIdentifierImpl loopVariable,
      Token inKeyword, ExpressionImpl iterator)
      : super(inKeyword, iterator) {
    _loopVariable = _becomeParentOf(loopVariable);
  }

  @override
  Token get beginToken => _loopVariable.beginToken;

  @override
  Iterable<SyntacticEntity> get childEntities => new ChildEntities()
    ..add(_loopVariable)
    ..addAll(super.childEntities);

  @override
  DeclaredIdentifier get loopVariable => _loopVariable;

  void set loopVariable(DeclaredIdentifier variable) {
    _loopVariable = _becomeParentOf(variable as DeclaredIdentifierImpl);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) =>
      visitor.visitForEachPartsWithDeclaration(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _loopVariable?.accept(visitor);
    super.visitChildren(visitor);
  }
}

class ForEachPartsWithIdentifierImpl extends ForEachPartsImpl
    implements ForEachPartsWithIdentifier {
  /**
   * The loop variable.
   */
  SimpleIdentifierImpl _identifier;

  /**
   * Initialize a newly created for-each statement whose loop control variable
   * is declared externally (outside the for-loop part).
   */
  ForEachPartsWithIdentifierImpl(
      SimpleIdentifierImpl identifier, Token inKeyword, ExpressionImpl iterator)
      : super(inKeyword, iterator) {
    _identifier = _becomeParentOf(identifier);
  }

  @override
  Token get beginToken => _identifier.beginToken;

  @override
  Iterable<SyntacticEntity> get childEntities => new ChildEntities()
    ..add(_identifier)
    ..addAll(super.childEntities);

  @override
  SimpleIdentifier get identifier => _identifier;

  void set identifier(SimpleIdentifier identifier) {
    _identifier = _becomeParentOf(identifier as SimpleIdentifierImpl);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) =>
      visitor.visitForEachPartsWithIdentifier(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _identifier?.accept(visitor);
    _iterable?.accept(visitor);
  }
}

/**
 * A for-each statement.
 *
 *    forEachStatement ::=
 *        'await'? 'for' '(' [DeclaredIdentifier] 'in' [Expression] ')' [Block]
 *      | 'await'? 'for' '(' [SimpleIdentifier] 'in' [Expression] ')' [Block]
 */
class ForEachStatementImpl extends StatementImpl implements ForEachStatement {
  /**
   * The token representing the 'await' keyword, or `null` if there is no
   * 'await' keyword.
   */
  @override
  Token awaitKeyword;

  /**
   * The token representing the 'for' keyword.
   */
  @override
  Token forKeyword;

  /**
   * The left parenthesis.
   */
  @override
  Token leftParenthesis;

  /**
   * The declaration of the loop variable, or `null` if the loop variable is a
   * simple identifier.
   */
  DeclaredIdentifierImpl _loopVariable;

  /**
   * The loop variable, or `null` if the loop variable is declared in the 'for'.
   */
  SimpleIdentifierImpl _identifier;

  /**
   * The token representing the 'in' keyword.
   */
  @override
  Token inKeyword;

  /**
   * The expression evaluated to produce the iterator.
   */
  ExpressionImpl _iterable;

  /**
   * The right parenthesis.
   */
  @override
  Token rightParenthesis;

  /**
   * The body of the loop.
   */
  StatementImpl _body;

  /**
   * Initialize a newly created for-each statement whose loop control variable
   * is declared internally (in the for-loop part). The [awaitKeyword] can be
   * `null` if this is not an asynchronous for loop.
   */
  ForEachStatementImpl.withDeclaration(
      this.awaitKeyword,
      this.forKeyword,
      this.leftParenthesis,
      DeclaredIdentifierImpl loopVariable,
      this.inKeyword,
      ExpressionImpl iterator,
      this.rightParenthesis,
      StatementImpl body) {
    _loopVariable = _becomeParentOf(loopVariable);
    _iterable = _becomeParentOf(iterator);
    _body = _becomeParentOf(body);
  }

  /**
   * Initialize a newly created for-each statement whose loop control variable
   * is declared outside the for loop. The [awaitKeyword] can be `null` if this
   * is not an asynchronous for loop.
   */
  ForEachStatementImpl.withReference(
      this.awaitKeyword,
      this.forKeyword,
      this.leftParenthesis,
      SimpleIdentifierImpl identifier,
      this.inKeyword,
      ExpressionImpl iterator,
      this.rightParenthesis,
      StatementImpl body) {
    _identifier = _becomeParentOf(identifier);
    _iterable = _becomeParentOf(iterator);
    _body = _becomeParentOf(body);
  }

  @override
  Token get beginToken => awaitKeyword ?? forKeyword;

  @override
  Statement get body => _body;

  @override
  void set body(Statement statement) {
    _body = _becomeParentOf(statement as StatementImpl);
  }

  @override
  Iterable<SyntacticEntity> get childEntities => new ChildEntities()
    ..add(awaitKeyword)
    ..add(forKeyword)
    ..add(leftParenthesis)
    ..add(_loopVariable)
    ..add(_identifier)
    ..add(inKeyword)
    ..add(_iterable)
    ..add(rightParenthesis)
    ..add(_body);

  @override
  Token get endToken => _body.endToken;

  @override
  SimpleIdentifier get identifier => _identifier;

  @override
  void set identifier(SimpleIdentifier identifier) {
    _identifier = _becomeParentOf(identifier as SimpleIdentifierImpl);
  }

  @override
  Expression get iterable => _iterable;

  @override
  void set iterable(Expression expression) {
    _iterable = _becomeParentOf(expression as ExpressionImpl);
  }

  @override
  DeclaredIdentifier get loopVariable => _loopVariable;

  @override
  void set loopVariable(DeclaredIdentifier variable) {
    _loopVariable = _becomeParentOf(variable as DeclaredIdentifierImpl);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitForEachStatement(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _loopVariable?.accept(visitor);
    _identifier?.accept(visitor);
    _iterable?.accept(visitor);
    _body?.accept(visitor);
  }
}

abstract class ForLoopPartsImpl extends AstNodeImpl implements ForLoopParts {}

/**
 * A node representing a parameter to a function.
 *
 *    formalParameter ::=
 *        [NormalFormalParameter]
 *      | [DefaultFormalParameter]
 */
abstract class FormalParameterImpl extends AstNodeImpl
    implements FormalParameter {
  @override
  ParameterElement get declaredElement {
    SimpleIdentifier identifier = this.identifier;
    if (identifier == null) {
      return null;
    }
    return identifier.staticElement as ParameterElement;
  }

  @deprecated
  @override
  ParameterElement get element => declaredElement;

  @override
  bool get isNamed => kind == ParameterKind.NAMED;

  @override
  bool get isOptional =>
      kind == ParameterKind.NAMED || kind == ParameterKind.POSITIONAL;

  @override
  bool get isOptionalPositional => kind == ParameterKind.POSITIONAL;

  @override
  bool get isPositional =>
      kind == ParameterKind.POSITIONAL || kind == ParameterKind.REQUIRED;

  @override
  bool get isRequired => kind == ParameterKind.REQUIRED;

  @override
  // Overridden to remove the 'deprecated' annotation.
  ParameterKind get kind;
}

/**
 * The formal parameter list of a method declaration, function declaration, or
 * function type alias.
 *
 * While the grammar requires all optional formal parameters to follow all of
 * the normal formal parameters and at most one grouping of optional formal
 * parameters, this class does not enforce those constraints. All parameters are
 * flattened into a single list, which can have any or all kinds of parameters
 * (normal, named, and positional) in any order.
 *
 *    formalParameterList ::=
 *        '(' ')'
 *      | '(' normalFormalParameters (',' optionalFormalParameters)? ')'
 *      | '(' optionalFormalParameters ')'
 *
 *    normalFormalParameters ::=
 *        [NormalFormalParameter] (',' [NormalFormalParameter])*
 *
 *    optionalFormalParameters ::=
 *        optionalPositionalFormalParameters
 *      | namedFormalParameters
 *
 *    optionalPositionalFormalParameters ::=
 *        '[' [DefaultFormalParameter] (',' [DefaultFormalParameter])* ']'
 *
 *    namedFormalParameters ::=
 *        '{' [DefaultFormalParameter] (',' [DefaultFormalParameter])* '}'
 */
class FormalParameterListImpl extends AstNodeImpl
    implements FormalParameterList {
  /**
   * The left parenthesis.
   */
  @override
  Token leftParenthesis;

  /**
   * The parameters associated with the method.
   */
  NodeList<FormalParameter> _parameters;

  /**
   * The left square bracket ('[') or left curly brace ('{') introducing the
   * optional parameters, or `null` if there are no optional parameters.
   */
  @override
  Token leftDelimiter;

  /**
   * The right square bracket (']') or right curly brace ('}') terminating the
   * optional parameters, or `null` if there are no optional parameters.
   */
  @override
  Token rightDelimiter;

  /**
   * The right parenthesis.
   */
  @override
  Token rightParenthesis;

  /**
   * Initialize a newly created parameter list. The list of [parameters] can be
   * `null` if there are no parameters. The [leftDelimiter] and [rightDelimiter]
   * can be `null` if there are no optional parameters.
   */
  FormalParameterListImpl(
      this.leftParenthesis,
      List<FormalParameter> parameters,
      this.leftDelimiter,
      this.rightDelimiter,
      this.rightParenthesis) {
    _parameters = new NodeListImpl<FormalParameter>(this, parameters);
  }

  @override
  Token get beginToken => leftParenthesis;

  @override
  Iterable<SyntacticEntity> get childEntities {
    // TODO(paulberry): include commas.
    ChildEntities result = new ChildEntities()..add(leftParenthesis);
    bool leftDelimiterNeeded = leftDelimiter != null;
    int length = _parameters.length;
    for (int i = 0; i < length; i++) {
      FormalParameter parameter = _parameters[i];
      if (leftDelimiterNeeded && leftDelimiter.offset < parameter.offset) {
        result.add(leftDelimiter);
        leftDelimiterNeeded = false;
      }
      result.add(parameter);
    }
    return result..add(rightDelimiter)..add(rightParenthesis);
  }

  @override
  Token get endToken => rightParenthesis;

  @override
  List<ParameterElement> get parameterElements {
    int count = _parameters.length;
    List<ParameterElement> types = new List<ParameterElement>(count);
    for (int i = 0; i < count; i++) {
      types[i] = _parameters[i].declaredElement;
    }
    return types;
  }

  @override
  NodeList<FormalParameter> get parameters => _parameters;

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitFormalParameterList(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _parameters.accept(visitor);
  }
}

mixin ForMixin on AstNodeImpl {
  Token awaitKeyword;

  Token forKeyword;

  Token leftParenthesis;

  ForLoopPartsImpl _forLoopParts;

  Token rightParenthesis;

  @override
  Token get beginToken => awaitKeyword ?? forKeyword;

  @override
  Iterable<SyntacticEntity> get childEntities => new ChildEntities()
    ..add(awaitKeyword)
    ..add(forKeyword)
    ..add(leftParenthesis)
    ..add(_forLoopParts)
    ..add(rightParenthesis);

  ForLoopParts get forLoopParts => _forLoopParts;

  void set forLoopParts(ForLoopParts forLoopParts) {
    _forLoopParts = _becomeParentOf(forLoopParts as ForLoopPartsImpl);
  }
}

abstract class ForPartsImpl extends ForLoopPartsImpl implements ForParts {
  @override
  Token leftSeparator;

  /**
   * The condition used to determine when to terminate the loop, or `null` if
   * there is no condition.
   */
  ExpressionImpl _condition;

  @override
  Token rightSeparator;

  /**
   * The list of expressions run after each execution of the loop body.
   */
  NodeList<Expression> _updaters;

  /**
   * Initialize a newly created for statement. Either the [variableList] or the
   * [initialization] must be `null`. Either the [condition] and the list of
   * [updaters] can be `null` if the loop does not have the corresponding
   * attribute.
   */
  ForPartsImpl(this.leftSeparator, ExpressionImpl condition,
      this.rightSeparator, List<Expression> updaters) {
    _condition = _becomeParentOf(condition);
    _updaters = new NodeListImpl<Expression>(this, updaters);
  }

  @override
  Token get beginToken => leftSeparator;

  @override
  Iterable<SyntacticEntity> get childEntities => new ChildEntities()
    ..add(leftSeparator)
    ..add(_condition)
    ..add(rightSeparator)
    ..addAll(_updaters);

  @override
  Expression get condition => _condition;

  void set condition(Expression expression) {
    _condition = _becomeParentOf(expression as ExpressionImpl);
  }

  @override
  Token get endToken => _updaters?.endToken ?? rightSeparator;

  @override
  NodeList<Expression> get updaters => _updaters;

  @override
  void visitChildren(AstVisitor visitor) {
    _condition?.accept(visitor);
    _updaters.accept(visitor);
  }
}

class ForPartsWithDeclarationsImpl extends ForPartsImpl
    implements ForPartsWithDeclarations {
  /**
   * The declaration of the loop variables, or `null` if there are no variables.
   * Note that a for statement cannot have both a variable list and an
   * initialization expression, but can validly have neither.
   */
  VariableDeclarationListImpl _variableList;

  /**
   * Initialize a newly created for statement. Both the [condition] and the list
   * of [updaters] can be `null` if the loop does not have the corresponding
   * attribute.
   */
  ForPartsWithDeclarationsImpl(
      VariableDeclarationListImpl variableList,
      Token leftSeparator,
      ExpressionImpl condition,
      Token rightSeparator,
      List<Expression> updaters)
      : super(leftSeparator, condition, rightSeparator, updaters) {
    _variableList = _becomeParentOf(variableList);
  }

  @override
  Token get beginToken => _variableList?.beginToken ?? super.beginToken;

  @override
  Iterable<SyntacticEntity> get childEntities => new ChildEntities()
    ..add(_variableList)
    ..addAll(super.childEntities);

  @override
  VariableDeclarationList get variables => _variableList;

  void set variables(VariableDeclarationList variableList) {
    _variableList =
        _becomeParentOf(variableList as VariableDeclarationListImpl);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) =>
      visitor.visitForPartsWithDeclarations(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _variableList?.accept(visitor);
    super.visitChildren(visitor);
  }
}

class ForPartsWithExpressionImpl extends ForPartsImpl
    implements ForPartsWithExpression {
  /**
   * The initialization expression, or `null` if there is no initialization
   * expression. Note that a for statement cannot have both a variable list and
   * an initialization expression, but can validly have neither.
   */
  ExpressionImpl _initialization;

  /**
   * Initialize a newly created for statement. Both the [condition] and the list
   * of [updaters] can be `null` if the loop does not have the corresponding
   * attribute.
   */
  ForPartsWithExpressionImpl(ExpressionImpl initialization, Token leftSeparator,
      ExpressionImpl condition, Token rightSeparator, List<Expression> updaters)
      : super(leftSeparator, condition, rightSeparator, updaters) {
    _initialization = _becomeParentOf(initialization);
  }

  @override
  Token get beginToken => initialization?.beginToken ?? super.beginToken;

  @override
  Iterable<SyntacticEntity> get childEntities => new ChildEntities()
    ..add(_initialization)
    ..addAll(super.childEntities);

  @override
  Expression get initialization => _initialization;

  void set initialization(Expression initialization) {
    _initialization = _becomeParentOf(initialization as ExpressionImpl);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) =>
      visitor.visitForPartsWithExpression(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _initialization?.accept(visitor);
    super.visitChildren(visitor);
  }
}

class ForStatement2Impl extends StatementImpl
    with ForMixin
    implements ForStatement2 {
  /**
   * The body of the loop.
   */
  StatementImpl _body;

  /**
   * Initialize a newly created for statement.
   */
  ForStatement2Impl(
      Token awaitKeyword,
      Token forKeyword,
      Token leftParenthesis,
      ForLoopPartsImpl forLoopParts,
      Token rightParenthesis,
      StatementImpl body) {
    this.awaitKeyword = awaitKeyword;
    this.forKeyword = forKeyword;
    this.leftParenthesis = leftParenthesis;
    _forLoopParts = _becomeParentOf(forLoopParts);
    this.rightParenthesis = rightParenthesis;
    _body = _becomeParentOf(body);
  }

  @override
  Statement get body => _body;

  void set body(Statement statement) {
    _body = _becomeParentOf(statement as StatementImpl);
  }

  @override
  Iterable<SyntacticEntity> get childEntities => new ChildEntities()
    ..addAll(super.childEntities)
    ..add(_body);

  @override
  Token get endToken => _body.endToken;

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitForStatement2(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _forLoopParts?.accept(visitor);
    _body?.accept(visitor);
  }
}

/**
 * A for statement.
 *
 *    forStatement ::=
 *        'for' '(' forLoopParts ')' [Statement]
 *
 *    forLoopParts ::=
 *        forInitializerStatement ';' [Expression]? ';' [Expression]?
 *
 *    forInitializerStatement ::=
 *        [DefaultFormalParameter]
 *      | [Expression]?
 */
class ForStatementImpl extends StatementImpl implements ForStatement {
  /**
   * The token representing the 'for' keyword.
   */
  @override
  Token forKeyword;

  /**
   * The left parenthesis.
   */
  @override
  Token leftParenthesis;

  /**
   * The declaration of the loop variables, or `null` if there are no variables.
   * Note that a for statement cannot have both a variable list and an
   * initialization expression, but can validly have neither.
   */
  VariableDeclarationListImpl _variableList;

  /**
   * The initialization expression, or `null` if there is no initialization
   * expression. Note that a for statement cannot have both a variable list and
   * an initialization expression, but can validly have neither.
   */
  ExpressionImpl _initialization;

  /**
   * The semicolon separating the initializer and the condition.
   */
  @override
  Token leftSeparator;

  /**
   * The condition used to determine when to terminate the loop, or `null` if
   * there is no condition.
   */
  ExpressionImpl _condition;

  /**
   * The semicolon separating the condition and the updater.
   */
  @override
  Token rightSeparator;

  /**
   * The list of expressions run after each execution of the loop body.
   */
  NodeList<Expression> _updaters;

  /**
   * The right parenthesis.
   */
  @override
  Token rightParenthesis;

  /**
   * The body of the loop.
   */
  StatementImpl _body;

  /**
   * Initialize a newly created for statement. Either the [variableList] or the
   * [initialization] must be `null`. Either the [condition] and the list of
   * [updaters] can be `null` if the loop does not have the corresponding
   * attribute.
   */
  ForStatementImpl(
      this.forKeyword,
      this.leftParenthesis,
      VariableDeclarationListImpl variableList,
      ExpressionImpl initialization,
      this.leftSeparator,
      ExpressionImpl condition,
      this.rightSeparator,
      List<Expression> updaters,
      this.rightParenthesis,
      StatementImpl body) {
    _variableList = _becomeParentOf(variableList);
    _initialization = _becomeParentOf(initialization);
    _condition = _becomeParentOf(condition);
    _updaters = new NodeListImpl<Expression>(this, updaters);
    _body = _becomeParentOf(body);
  }

  @override
  Token get beginToken => forKeyword;

  @override
  Statement get body => _body;

  @override
  void set body(Statement statement) {
    _body = _becomeParentOf(statement as StatementImpl);
  }

  @override
  Iterable<SyntacticEntity> get childEntities => new ChildEntities()
    ..add(forKeyword)
    ..add(leftParenthesis)
    ..add(_variableList)
    ..add(_initialization)
    ..add(leftSeparator)
    ..add(_condition)
    ..add(rightSeparator)
    ..addAll(_updaters)
    ..add(rightParenthesis)
    ..add(_body);

  @override
  Expression get condition => _condition;

  @override
  void set condition(Expression expression) {
    _condition = _becomeParentOf(expression as ExpressionImpl);
  }

  @override
  Token get endToken => _body.endToken;

  @override
  Expression get initialization => _initialization;

  @override
  void set initialization(Expression initialization) {
    _initialization = _becomeParentOf(initialization as ExpressionImpl);
  }

  @override
  NodeList<Expression> get updaters => _updaters;

  @override
  VariableDeclarationList get variables => _variableList;

  @override
  void set variables(VariableDeclarationList variableList) {
    _variableList =
        _becomeParentOf(variableList as VariableDeclarationListImpl);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitForStatement(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _variableList?.accept(visitor);
    _initialization?.accept(visitor);
    _condition?.accept(visitor);
    _updaters.accept(visitor);
    _body?.accept(visitor);
  }
}

/**
 * A node representing the body of a function or method.
 *
 *    functionBody ::=
 *        [BlockFunctionBody]
 *      | [EmptyFunctionBody]
 *      | [ExpressionFunctionBody]
 */
abstract class FunctionBodyImpl extends AstNodeImpl implements FunctionBody {
  /**
   * Additional information about local variables and parameters that are
   * declared within this function body or any enclosing function body.  `null`
   * if resolution has not yet been performed.
   */
  LocalVariableInfo localVariableInfo;

  /**
   * Return `true` if this function body is asynchronous.
   */
  @override
  bool get isAsynchronous => false;

  /**
   * Return `true` if this function body is a generator.
   */
  @override
  bool get isGenerator => false;

  /**
   * Return `true` if this function body is synchronous.
   */
  @override
  bool get isSynchronous => true;

  /**
   * Return the token representing the 'async' or 'sync' keyword, or `null` if
   * there is no such keyword.
   */
  @override
  Token get keyword => null;

  /**
   * Return the star following the 'async' or 'sync' keyword, or `null` if there
   * is no star.
   */
  @override
  Token get star => null;

  @override
  bool isPotentiallyMutatedInClosure(VariableElement variable) {
    if (localVariableInfo == null) {
      throw new StateError('Resolution has not yet been performed');
    }
    return localVariableInfo.potentiallyMutatedInClosure.contains(variable);
  }

  @override
  bool isPotentiallyMutatedInScope(VariableElement variable) {
    if (localVariableInfo == null) {
      throw new StateError('Resolution has not yet been performed');
    }
    return localVariableInfo.potentiallyMutatedInScope.contains(variable);
  }
}

/**
 * A top-level declaration.
 *
 *    functionDeclaration ::=
 *        'external' functionSignature
 *      | functionSignature [FunctionBody]
 *
 *    functionSignature ::=
 *        [Type]? ('get' | 'set')? [SimpleIdentifier] [FormalParameterList]
 */
class FunctionDeclarationImpl extends NamedCompilationUnitMemberImpl
    implements FunctionDeclaration {
  /**
   * The token representing the 'external' keyword, or `null` if this is not an
   * external function.
   */
  @override
  Token externalKeyword;

  /**
   * The return type of the function, or `null` if no return type was declared.
   */
  TypeAnnotationImpl _returnType;

  /**
   * The token representing the 'get' or 'set' keyword, or `null` if this is a
   * function declaration rather than a property declaration.
   */
  @override
  Token propertyKeyword;

  /**
   * The function expression being wrapped.
   */
  FunctionExpressionImpl _functionExpression;

  /**
   * Initialize a newly created function declaration. Either or both of the
   * [comment] and [metadata] can be `null` if the function does not have the
   * corresponding attribute. The [externalKeyword] can be `null` if the
   * function is not an external function. The [returnType] can be `null` if no
   * return type was specified. The [propertyKeyword] can be `null` if the
   * function is neither a getter or a setter.
   */
  FunctionDeclarationImpl(
      CommentImpl comment,
      List<Annotation> metadata,
      this.externalKeyword,
      TypeAnnotationImpl returnType,
      this.propertyKeyword,
      SimpleIdentifierImpl name,
      FunctionExpressionImpl functionExpression)
      : super(comment, metadata, name) {
    _returnType = _becomeParentOf(returnType);
    _functionExpression = _becomeParentOf(functionExpression);
  }

  @override
  Iterable<SyntacticEntity> get childEntities => super._childEntities
    ..add(externalKeyword)
    ..add(_returnType)
    ..add(propertyKeyword)
    ..add(_name)
    ..add(_functionExpression);

  @override
  ExecutableElement get declaredElement =>
      _name?.staticElement as ExecutableElement;

  @deprecated
  @override
  ExecutableElement get element => declaredElement;

  @override
  Token get endToken => _functionExpression.endToken;

  @override
  Token get firstTokenAfterCommentAndMetadata {
    if (externalKeyword != null) {
      return externalKeyword;
    } else if (_returnType != null) {
      return _returnType.beginToken;
    } else if (propertyKeyword != null) {
      return propertyKeyword;
    } else if (_name != null) {
      return _name.beginToken;
    }
    return _functionExpression.beginToken;
  }

  @override
  FunctionExpression get functionExpression => _functionExpression;

  @override
  void set functionExpression(FunctionExpression functionExpression) {
    _functionExpression =
        _becomeParentOf(functionExpression as FunctionExpressionImpl);
  }

  @override
  bool get isGetter => propertyKeyword?.keyword == Keyword.GET;

  @override
  bool get isSetter => propertyKeyword?.keyword == Keyword.SET;

  @override
  TypeAnnotation get returnType => _returnType;

  @override
  void set returnType(TypeAnnotation type) {
    _returnType = _becomeParentOf(type as TypeAnnotationImpl);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitFunctionDeclaration(this);

  @override
  void visitChildren(AstVisitor visitor) {
    super.visitChildren(visitor);
    _returnType?.accept(visitor);
    _name?.accept(visitor);
    _functionExpression?.accept(visitor);
  }
}

/**
 * A [FunctionDeclaration] used as a statement.
 */
class FunctionDeclarationStatementImpl extends StatementImpl
    implements FunctionDeclarationStatement {
  /**
   * The function declaration being wrapped.
   */
  FunctionDeclarationImpl _functionDeclaration;

  /**
   * Initialize a newly created function declaration statement.
   */
  FunctionDeclarationStatementImpl(
      FunctionDeclarationImpl functionDeclaration) {
    _functionDeclaration = _becomeParentOf(functionDeclaration);
  }

  @override
  Token get beginToken => _functionDeclaration.beginToken;

  @override
  Iterable<SyntacticEntity> get childEntities =>
      new ChildEntities()..add(_functionDeclaration);

  @override
  Token get endToken => _functionDeclaration.endToken;

  @override
  FunctionDeclaration get functionDeclaration => _functionDeclaration;

  @override
  void set functionDeclaration(FunctionDeclaration functionDeclaration) {
    _functionDeclaration =
        _becomeParentOf(functionDeclaration as FunctionDeclarationImpl);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) =>
      visitor.visitFunctionDeclarationStatement(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _functionDeclaration?.accept(visitor);
  }
}

/**
 * A function expression.
 *
 *    functionExpression ::=
 *        [TypeParameterList]? [FormalParameterList] [FunctionBody]
 */
class FunctionExpressionImpl extends ExpressionImpl
    implements FunctionExpression {
  /**
   * The type parameters associated with the method, or `null` if the method is
   * not a generic method.
   */
  TypeParameterListImpl _typeParameters;

  /**
   * The parameters associated with the function.
   */
  FormalParameterListImpl _parameters;

  /**
   * The body of the function, or `null` if this is an external function.
   */
  FunctionBodyImpl _body;

  @override
  ExecutableElement declaredElement;

  /**
   * Initialize a newly created function declaration.
   */
  FunctionExpressionImpl(TypeParameterListImpl typeParameters,
      FormalParameterListImpl parameters, FunctionBodyImpl body) {
    _typeParameters = _becomeParentOf(typeParameters);
    _parameters = _becomeParentOf(parameters);
    _body = _becomeParentOf(body);
  }

  @override
  Token get beginToken {
    if (_typeParameters != null) {
      return _typeParameters.beginToken;
    } else if (_parameters != null) {
      return _parameters.beginToken;
    } else if (_body != null) {
      return _body.beginToken;
    }
    // This should never be reached because external functions must be named,
    // hence either the body or the name should be non-null.
    throw new StateError("Non-external functions must have a body");
  }

  @override
  FunctionBody get body => _body;

  @override
  void set body(FunctionBody functionBody) {
    _body = _becomeParentOf(functionBody as FunctionBodyImpl);
  }

  @override
  Iterable<SyntacticEntity> get childEntities =>
      new ChildEntities()..add(_parameters)..add(_body);

  @deprecated
  @override
  ExecutableElement get element => declaredElement;

  @deprecated
  @override
  set element(ExecutableElement element) {
    declaredElement = element;
  }

  @override
  Token get endToken {
    if (_body != null) {
      return _body.endToken;
    } else if (_parameters != null) {
      return _parameters.endToken;
    }
    // This should never be reached because external functions must be named,
    // hence either the body or the name should be non-null.
    throw new StateError("Non-external functions must have a body");
  }

  @override
  FormalParameterList get parameters => _parameters;

  @override
  void set parameters(FormalParameterList parameters) {
    _parameters = _becomeParentOf(parameters as FormalParameterListImpl);
  }

  @override
  int get precedence => 16;

  @override
  TypeParameterList get typeParameters => _typeParameters;

  @override
  void set typeParameters(TypeParameterList typeParameters) {
    _typeParameters = _becomeParentOf(typeParameters as TypeParameterListImpl);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitFunctionExpression(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _typeParameters?.accept(visitor);
    _parameters?.accept(visitor);
    _body?.accept(visitor);
  }
}

/**
 * The invocation of a function resulting from evaluating an expression.
 * Invocations of methods and other forms of functions are represented by
 * [MethodInvocation] nodes. Invocations of getters and setters are represented
 * by either [PrefixedIdentifier] or [PropertyAccess] nodes.
 *
 *    functionExpressionInvocation ::=
 *        [Expression] [TypeArgumentList]? [ArgumentList]
 */
class FunctionExpressionInvocationImpl extends InvocationExpressionImpl
    implements FunctionExpressionInvocation {
  /**
   * The expression producing the function being invoked.
   */
  ExpressionImpl _function;

  /**
   * The element associated with the function being invoked based on static type
   * information, or `null` if the AST structure has not been resolved or the
   * function could not be resolved.
   */
  @override
  ExecutableElement staticElement;

  /**
   * Initialize a newly created function expression invocation.
   */
  FunctionExpressionInvocationImpl(ExpressionImpl function,
      TypeArgumentListImpl typeArguments, ArgumentListImpl argumentList)
      : super(typeArguments, argumentList) {
    _function = _becomeParentOf(function);
  }

  @override
  Token get beginToken => _function.beginToken;

  @override
  @deprecated
  ExecutableElement get bestElement => staticElement;

  @override
  Iterable<SyntacticEntity> get childEntities =>
      new ChildEntities()..add(_function)..add(_argumentList);

  @override
  Token get endToken => _argumentList.endToken;

  @override
  Expression get function => _function;

  @override
  void set function(Expression expression) {
    _function = _becomeParentOf(expression as ExpressionImpl);
  }

  @override
  int get precedence => 15;

  @deprecated
  @override
  ExecutableElement get propagatedElement => null;

  @deprecated
  @override
  set propagatedElement(ExecutableElement element) {}

  @override
  E accept<E>(AstVisitor<E> visitor) =>
      visitor.visitFunctionExpressionInvocation(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _function?.accept(visitor);
    _typeArguments?.accept(visitor);
    _argumentList?.accept(visitor);
  }
}

/**
 * A function type alias.
 *
 *    functionTypeAlias ::=
 *        functionPrefix [TypeParameterList]? [FormalParameterList] ';'
 *
 *    functionPrefix ::=
 *        [TypeName]? [SimpleIdentifier]
 */
class FunctionTypeAliasImpl extends TypeAliasImpl implements FunctionTypeAlias {
  /**
   * The name of the return type of the function type being defined, or `null`
   * if no return type was given.
   */
  TypeAnnotationImpl _returnType;

  /**
   * The type parameters for the function type, or `null` if the function type
   * does not have any type parameters.
   */
  TypeParameterListImpl _typeParameters;

  /**
   * The parameters associated with the function type.
   */
  FormalParameterListImpl _parameters;

  /**
   * Initialize a newly created function type alias. Either or both of the
   * [comment] and [metadata] can be `null` if the function does not have the
   * corresponding attribute. The [returnType] can be `null` if no return type
   * was specified. The [typeParameters] can be `null` if the function has no
   * type parameters.
   */
  FunctionTypeAliasImpl(
      CommentImpl comment,
      List<Annotation> metadata,
      Token keyword,
      TypeAnnotationImpl returnType,
      SimpleIdentifierImpl name,
      TypeParameterListImpl typeParameters,
      FormalParameterListImpl parameters,
      Token semicolon)
      : super(comment, metadata, keyword, name, semicolon) {
    _returnType = _becomeParentOf(returnType);
    _typeParameters = _becomeParentOf(typeParameters);
    _parameters = _becomeParentOf(parameters);
  }

  @override
  Iterable<SyntacticEntity> get childEntities => super._childEntities
    ..add(typedefKeyword)
    ..add(_returnType)
    ..add(_name)
    ..add(_typeParameters)
    ..add(_parameters)
    ..add(semicolon);

  @override
  FunctionTypeAliasElement get declaredElement =>
      _name?.staticElement as FunctionTypeAliasElement;

  @deprecated
  @override
  FunctionTypeAliasElement get element => declaredElement;

  @override
  FormalParameterList get parameters => _parameters;

  @override
  void set parameters(FormalParameterList parameters) {
    _parameters = _becomeParentOf(parameters as FormalParameterListImpl);
  }

  @override
  TypeAnnotation get returnType => _returnType;

  @override
  void set returnType(TypeAnnotation type) {
    _returnType = _becomeParentOf(type as TypeAnnotationImpl);
  }

  @override
  TypeParameterList get typeParameters => _typeParameters;

  @override
  void set typeParameters(TypeParameterList typeParameters) {
    _typeParameters = _becomeParentOf(typeParameters as TypeParameterListImpl);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitFunctionTypeAlias(this);

  @override
  void visitChildren(AstVisitor visitor) {
    super.visitChildren(visitor);
    _returnType?.accept(visitor);
    _name?.accept(visitor);
    _typeParameters?.accept(visitor);
    _parameters?.accept(visitor);
  }
}

/**
 * A function-typed formal parameter.
 *
 *    functionSignature ::=
 *        [TypeName]? [SimpleIdentifier] [TypeParameterList]? [FormalParameterList]
 */
class FunctionTypedFormalParameterImpl extends NormalFormalParameterImpl
    implements FunctionTypedFormalParameter {
  /**
   * The return type of the function, or `null` if the function does not have a
   * return type.
   */
  TypeAnnotationImpl _returnType;

  /**
   * The type parameters associated with the function, or `null` if the function
   * is not a generic function.
   */
  TypeParameterListImpl _typeParameters;

  /**
   * The parameters of the function-typed parameter.
   */
  FormalParameterListImpl _parameters;

  /**
   * Initialize a newly created formal parameter. Either or both of the
   * [comment] and [metadata] can be `null` if the parameter does not have the
   * corresponding attribute. The [returnType] can be `null` if no return type
   * was specified.
   */
  FunctionTypedFormalParameterImpl(
      CommentImpl comment,
      List<Annotation> metadata,
      Token covariantKeyword,
      TypeAnnotationImpl returnType,
      SimpleIdentifierImpl identifier,
      TypeParameterListImpl typeParameters,
      FormalParameterListImpl parameters)
      : super(comment, metadata, covariantKeyword, identifier) {
    _returnType = _becomeParentOf(returnType);
    _typeParameters = _becomeParentOf(typeParameters);
    _parameters = _becomeParentOf(parameters);
  }

  @override
  Token get beginToken =>
      this.metadata.beginToken ??
      covariantKeyword ??
      _returnType?.beginToken ??
      identifier?.beginToken;

  @override
  Iterable<SyntacticEntity> get childEntities =>
      super._childEntities..add(_returnType)..add(identifier)..add(parameters);

  @override
  Token get endToken => _parameters.endToken;

  @override
  bool get isConst => false;

  @override
  bool get isFinal => false;

  @override
  FormalParameterList get parameters => _parameters;

  @override
  void set parameters(FormalParameterList parameters) {
    _parameters = _becomeParentOf(parameters as FormalParameterListImpl);
  }

  @override
  TypeAnnotation get returnType => _returnType;

  @override
  void set returnType(TypeAnnotation type) {
    _returnType = _becomeParentOf(type as TypeAnnotationImpl);
  }

  @override
  TypeParameterList get typeParameters => _typeParameters;

  @override
  void set typeParameters(TypeParameterList typeParameters) {
    _typeParameters = _becomeParentOf(typeParameters as TypeParameterListImpl);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) =>
      visitor.visitFunctionTypedFormalParameter(this);

  @override
  void visitChildren(AstVisitor visitor) {
    super.visitChildren(visitor);
    _returnType?.accept(visitor);
    identifier?.accept(visitor);
    _typeParameters?.accept(visitor);
    _parameters?.accept(visitor);
  }
}

/**
 * An anonymous function type.
 *
 *    functionType ::=
 *        [TypeAnnotation]? 'Function' [TypeParameterList]? [FormalParameterList]
 *
 * where the FormalParameterList is being used to represent the following
 * grammar, despite the fact that FormalParameterList can represent a much
 * larger grammar than the one below. This is done in order to simplify the
 * implementation.
 *
 *    parameterTypeList ::=
 *        () |
 *        ( normalParameterTypes ,? ) |
 *        ( normalParameterTypes , optionalParameterTypes ) |
 *        ( optionalParameterTypes )
 *    namedParameterTypes ::=
 *        { namedParameterType (, namedParameterType)* ,? }
 *    namedParameterType ::=
 *        [TypeAnnotation]? [SimpleIdentifier]
 *    normalParameterTypes ::=
 *        normalParameterType (, normalParameterType)*
 *    normalParameterType ::=
 *        [TypeAnnotation] [SimpleIdentifier]?
 *    optionalParameterTypes ::=
 *        optionalPositionalParameterTypes | namedParameterTypes
 *    optionalPositionalParameterTypes ::=
 *        [ normalParameterTypes ,? ]
 */
class GenericFunctionTypeImpl extends TypeAnnotationImpl
    implements GenericFunctionType {
  /**
   * The name of the return type of the function type being defined, or
   * `null` if no return type was given.
   */
  TypeAnnotationImpl _returnType;

  @override
  Token functionKeyword;

  /**
   * The type parameters for the function type, or `null` if the function type
   * does not have any type parameters.
   */
  TypeParameterListImpl _typeParameters;

  /**
   * The parameters associated with the function type.
   */
  FormalParameterListImpl _parameters;

  @override
  Token question;

  @override
  DartType type;

  /**
   * Initialize a newly created generic function type.
   */
  GenericFunctionTypeImpl(TypeAnnotationImpl returnType, this.functionKeyword,
      TypeParameterListImpl typeParameters, FormalParameterListImpl parameters,
      {this.question}) {
    _returnType = _becomeParentOf(returnType);
    _typeParameters = _becomeParentOf(typeParameters);
    _parameters = _becomeParentOf(parameters);
  }

  @override
  Token get beginToken => _returnType?.beginToken ?? functionKeyword;

  @override
  Iterable<SyntacticEntity> get childEntities => new ChildEntities()
    ..add(_returnType)
    ..add(functionKeyword)
    ..add(_typeParameters)
    ..add(_parameters)
    ..add(question);

  @override
  Token get endToken => question ?? _parameters.endToken;

  @override
  FormalParameterList get parameters => _parameters;

  @override
  void set parameters(FormalParameterList parameters) {
    _parameters = _becomeParentOf(parameters as FormalParameterListImpl);
  }

  @override
  TypeAnnotation get returnType => _returnType;

  @override
  void set returnType(TypeAnnotation type) {
    _returnType = _becomeParentOf(type as TypeAnnotationImpl);
  }

  /**
   * Return the type parameters for the function type, or `null` if the function
   * type does not have any type parameters.
   */
  TypeParameterList get typeParameters => _typeParameters;

  /**
   * Set the type parameters for the function type to the given list of
   * [typeParameters].
   */
  void set typeParameters(TypeParameterList typeParameters) {
    _typeParameters = _becomeParentOf(typeParameters as TypeParameterListImpl);
  }

  // TODO: implement type
  @override
  E accept<E>(AstVisitor<E> visitor) {
    return visitor.visitGenericFunctionType(this);
  }

  @override
  void visitChildren(AstVisitor visitor) {
    _returnType?.accept(visitor);
    _typeParameters?.accept(visitor);
    _parameters?.accept(visitor);
  }
}

/**
 * A generic type alias.
 *
 *    functionTypeAlias ::=
 *        metadata 'typedef' [SimpleIdentifier] [TypeParameterList]? = [FunctionType] ';'
 */
class GenericTypeAliasImpl extends TypeAliasImpl implements GenericTypeAlias {
  /**
   * The type parameters for the function type, or `null` if the function
   * type does not have any type parameters.
   */
  TypeParameterListImpl _typeParameters;

  @override
  Token equals;

  /**
   * The type of function being defined by the alias.
   */
  GenericFunctionTypeImpl _functionType;

  /**
   * Returns a newly created generic type alias. Either or both of the
   * [comment] and [metadata] can be `null` if the variable list does not have
   * the corresponding attribute. The [typeParameters] can be `null` if there
   * are no type parameters.
   */
  GenericTypeAliasImpl(
      CommentImpl comment,
      List<Annotation> metadata,
      Token typedefToken,
      SimpleIdentifierImpl name,
      TypeParameterListImpl typeParameters,
      this.equals,
      GenericFunctionTypeImpl functionType,
      Token semicolon)
      : super(comment, metadata, typedefToken, name, semicolon) {
    _typeParameters = _becomeParentOf(typeParameters);
    _functionType = _becomeParentOf(functionType);
  }

  @override
  Iterable<SyntacticEntity> get childEntities => new ChildEntities()
    ..addAll(metadata)
    ..add(typedefKeyword)
    ..add(name)
    ..add(_typeParameters)
    ..add(equals)
    ..add(_functionType);

  @override
  Element get declaredElement => name.staticElement;

  @deprecated
  @override
  Element get element => declaredElement;

  @override
  GenericFunctionType get functionType => _functionType;

  @override
  void set functionType(GenericFunctionType functionType) {
    _functionType = _becomeParentOf(functionType as GenericFunctionTypeImpl);
  }

  @override
  TypeParameterList get typeParameters => _typeParameters;

  @override
  void set typeParameters(TypeParameterList typeParameters) {
    _typeParameters = _becomeParentOf(typeParameters as TypeParameterListImpl);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) {
    return visitor.visitGenericTypeAlias(this);
  }

  @override
  void visitChildren(AstVisitor visitor) {
    super.visitChildren(visitor);
    name?.accept(visitor);
    _typeParameters?.accept(visitor);
    _functionType?.accept(visitor);
  }
}

/**
 * A combinator that restricts the names being imported to those that are not in
 * a given list.
 *
 *    hideCombinator ::=
 *        'hide' [SimpleIdentifier] (',' [SimpleIdentifier])*
 */
class HideCombinatorImpl extends CombinatorImpl implements HideCombinator {
  /**
   * The list of names from the library that are hidden by this combinator.
   */
  NodeList<SimpleIdentifier> _hiddenNames;

  /**
   * Initialize a newly created import show combinator.
   */
  HideCombinatorImpl(Token keyword, List<SimpleIdentifier> hiddenNames)
      : super(keyword) {
    _hiddenNames = new NodeListImpl<SimpleIdentifier>(this, hiddenNames);
  }

  @override
  Iterable<SyntacticEntity> get childEntities => new ChildEntities()
    ..add(keyword)
    ..addAll(_hiddenNames);

  @override
  Token get endToken => _hiddenNames.endToken;

  @override
  NodeList<SimpleIdentifier> get hiddenNames => _hiddenNames;

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitHideCombinator(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _hiddenNames.accept(visitor);
  }
}

/**
 * A node that represents an identifier.
 *
 *    identifier ::=
 *        [SimpleIdentifier]
 *      | [PrefixedIdentifier]
 */
abstract class IdentifierImpl extends ExpressionImpl implements Identifier {
  /**
   * Return the best element available for this operator. If resolution was able
   * to find a better element based on type propagation, that element will be
   * returned. Otherwise, the element found using the result of static analysis
   * will be returned. If resolution has not been performed, then `null` will be
   * returned.
   */
  @override
  @deprecated
  Element get bestElement;

  @override
  bool get isAssignable => true;
}

mixin IfMixin on AstNodeImpl {
  Token ifKeyword;

  Token leftParenthesis;

  /**
   * The condition used to determine which of the branches is executed next.
   */
  ExpressionImpl _condition;

  Token rightParenthesis;

  Token elseKeyword;

  @override
  Token get beginToken => ifKeyword;

  @override
  Iterable<SyntacticEntity> get childEntities => new ChildEntities()
    ..add(ifKeyword)
    ..add(leftParenthesis)
    ..add(_condition)
    ..add(rightParenthesis)
    ..add(elseKeyword);

  Expression get condition => _condition;

  void set condition(Expression expression) {
    _condition = _becomeParentOf(expression as ExpressionImpl);
  }

  @override
  void visitChildren(AstVisitor visitor) {
    _condition?.accept(visitor);
  }
}

/**
 * An if statement.
 *
 *    ifStatement ::=
 *        'if' '(' [Expression] ')' [Statement] ('else' [Statement])?
 */
class IfStatementImpl extends StatementImpl implements IfStatement {
  /**
   * The token representing the 'if' keyword.
   */
  @override
  Token ifKeyword;

  /**
   * The left parenthesis.
   */
  @override
  Token leftParenthesis;

  /**
   * The condition used to determine which of the statements is executed next.
   */
  ExpressionImpl _condition;

  /**
   * The right parenthesis.
   */
  @override
  Token rightParenthesis;

  /**
   * The statement that is executed if the condition evaluates to `true`.
   */
  StatementImpl _thenStatement;

  /**
   * The token representing the 'else' keyword, or `null` if there is no else
   * statement.
   */
  @override
  Token elseKeyword;

  /**
   * The statement that is executed if the condition evaluates to `false`, or
   * `null` if there is no else statement.
   */
  StatementImpl _elseStatement;

  /**
   * Initialize a newly created if statement. The [elseKeyword] and
   * [elseStatement] can be `null` if there is no else clause.
   */
  IfStatementImpl(
      this.ifKeyword,
      this.leftParenthesis,
      ExpressionImpl condition,
      this.rightParenthesis,
      StatementImpl thenStatement,
      this.elseKeyword,
      StatementImpl elseStatement) {
    _condition = _becomeParentOf(condition);
    _thenStatement = _becomeParentOf(thenStatement);
    _elseStatement = _becomeParentOf(elseStatement);
  }

  @override
  Token get beginToken => ifKeyword;

  @override
  Iterable<SyntacticEntity> get childEntities => new ChildEntities()
    ..add(ifKeyword)
    ..add(leftParenthesis)
    ..add(_condition)
    ..add(rightParenthesis)
    ..add(_thenStatement)
    ..add(elseKeyword)
    ..add(_elseStatement);

  @override
  Expression get condition => _condition;

  @override
  void set condition(Expression expression) {
    _condition = _becomeParentOf(expression as ExpressionImpl);
  }

  @override
  Statement get elseStatement => _elseStatement;

  @override
  void set elseStatement(Statement statement) {
    _elseStatement = _becomeParentOf(statement as StatementImpl);
  }

  @override
  Token get endToken {
    if (_elseStatement != null) {
      return _elseStatement.endToken;
    }
    return _thenStatement.endToken;
  }

  @override
  Statement get thenStatement => _thenStatement;

  @override
  void set thenStatement(Statement statement) {
    _thenStatement = _becomeParentOf(statement as StatementImpl);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitIfStatement(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _condition?.accept(visitor);
    _thenStatement?.accept(visitor);
    _elseStatement?.accept(visitor);
  }
}

/**
 * The "implements" clause in an class declaration.
 *
 *    implementsClause ::=
 *        'implements' [TypeName] (',' [TypeName])*
 */
class ImplementsClauseImpl extends AstNodeImpl implements ImplementsClause {
  /**
   * The token representing the 'implements' keyword.
   */
  @override
  Token implementsKeyword;

  /**
   * The interfaces that are being implemented.
   */
  NodeList<TypeName> _interfaces;

  /**
   * Initialize a newly created implements clause.
   */
  ImplementsClauseImpl(this.implementsKeyword, List<TypeName> interfaces) {
    _interfaces = new NodeListImpl<TypeName>(this, interfaces);
  }

  @override
  Token get beginToken => implementsKeyword;

  @override
  // TODO(paulberry): add commas.
  Iterable<SyntacticEntity> get childEntities => new ChildEntities()
    ..add(implementsKeyword)
    ..addAll(interfaces);

  @override
  Token get endToken => _interfaces.endToken;

  @override
  NodeList<TypeName> get interfaces => _interfaces;

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitImplementsClause(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _interfaces.accept(visitor);
  }
}

/**
 * An import directive.
 *
 *    importDirective ::=
 *        [Annotation] 'import' [StringLiteral] ('as' identifier)? [Combinator]* ';'
 *      | [Annotation] 'import' [StringLiteral] 'deferred' 'as' identifier [Combinator]* ';'
 */
class ImportDirectiveImpl extends NamespaceDirectiveImpl
    implements ImportDirective {
  /**
   * The token representing the 'deferred' keyword, or `null` if the imported is
   * not deferred.
   */
  Token deferredKeyword;

  /**
   * The token representing the 'as' keyword, or `null` if the imported names are
   * not prefixed.
   */
  @override
  Token asKeyword;

  /**
   * The prefix to be used with the imported names, or `null` if the imported
   * names are not prefixed.
   */
  SimpleIdentifierImpl _prefix;

  /**
   * Initialize a newly created import directive. Either or both of the
   * [comment] and [metadata] can be `null` if the function does not have the
   * corresponding attribute. The [deferredKeyword] can be `null` if the import
   * is not deferred. The [asKeyword] and [prefix] can be `null` if the import
   * does not specify a prefix. The list of [combinators] can be `null` if there
   * are no combinators.
   */
  ImportDirectiveImpl(
      CommentImpl comment,
      List<Annotation> metadata,
      Token keyword,
      StringLiteralImpl libraryUri,
      List<Configuration> configurations,
      this.deferredKeyword,
      this.asKeyword,
      SimpleIdentifierImpl prefix,
      List<Combinator> combinators,
      Token semicolon)
      : super(comment, metadata, keyword, libraryUri, configurations,
            combinators, semicolon) {
    _prefix = _becomeParentOf(prefix);
  }

  @override
  Iterable<SyntacticEntity> get childEntities => super._childEntities
    ..add(_uri)
    ..add(deferredKeyword)
    ..add(asKeyword)
    ..add(_prefix)
    ..addAll(combinators)
    ..add(semicolon);

  @override
  ImportElement get element => super.element as ImportElement;

  @override
  SimpleIdentifier get prefix => _prefix;

  @override
  void set prefix(SimpleIdentifier identifier) {
    _prefix = _becomeParentOf(identifier as SimpleIdentifierImpl);
  }

  @override
  LibraryElement get uriElement {
    ImportElement element = this.element;
    if (element == null) {
      return null;
    }
    return element.importedLibrary;
  }

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitImportDirective(this);

  @override
  void visitChildren(AstVisitor visitor) {
    super.visitChildren(visitor);
    _prefix?.accept(visitor);
    combinators.accept(visitor);
  }
}

/**
 * An index expression.
 *
 *    indexExpression ::=
 *        [Expression] '[' [Expression] ']'
 */
class IndexExpressionImpl extends ExpressionImpl implements IndexExpression {
  /**
   * The expression used to compute the object being indexed, or `null` if this
   * index expression is part of a cascade expression.
   */
  ExpressionImpl _target;

  /**
   * The period ("..") before a cascaded index expression, or `null` if this
   * index expression is not part of a cascade expression.
   */
  @override
  Token period;

  /**
   * The left square bracket.
   */
  @override
  Token leftBracket;

  /**
   * The expression used to compute the index.
   */
  ExpressionImpl _index;

  /**
   * The right square bracket.
   */
  @override
  Token rightBracket;

  /**
   * The element associated with the operator based on the static type of the
   * target, or `null` if the AST structure has not been resolved or if the
   * operator could not be resolved.
   */
  @override
  MethodElement staticElement;

  /**
   * If this expression is both in a getter and setter context, the
   * [AuxiliaryElements] will be set to hold onto the static element from the
   * getter context.
   */
  AuxiliaryElements auxiliaryElements = null;

  /**
   * Initialize a newly created index expression.
   */
  IndexExpressionImpl.forCascade(
      this.period, this.leftBracket, ExpressionImpl index, this.rightBracket) {
    _index = _becomeParentOf(index);
  }

  /**
   * Initialize a newly created index expression.
   */
  IndexExpressionImpl.forTarget(ExpressionImpl target, this.leftBracket,
      ExpressionImpl index, this.rightBracket) {
    _target = _becomeParentOf(target);
    _index = _becomeParentOf(index);
  }

  @override
  Token get beginToken {
    if (_target != null) {
      return _target.beginToken;
    }
    return period;
  }

  @override
  @deprecated
  MethodElement get bestElement => staticElement;

  @override
  Iterable<SyntacticEntity> get childEntities => new ChildEntities()
    ..add(_target)
    ..add(period)
    ..add(leftBracket)
    ..add(_index)
    ..add(rightBracket);

  @override
  Token get endToken => rightBracket;

  @override
  Expression get index => _index;

  @override
  void set index(Expression expression) {
    _index = _becomeParentOf(expression as ExpressionImpl);
  }

  @override
  bool get isAssignable => true;

  @override
  bool get isCascaded => period != null;

  @override
  int get precedence => 15;

  @deprecated
  @override
  MethodElement get propagatedElement => null;

  @deprecated
  @override
  set propagatedElement(MethodElement element) {}

  @override
  Expression get realTarget {
    if (isCascaded) {
      AstNode ancestor = parent;
      while (ancestor is! CascadeExpression) {
        if (ancestor == null) {
          return _target;
        }
        ancestor = ancestor.parent;
      }
      return (ancestor as CascadeExpression).target;
    }
    return _target;
  }

  @override
  Expression get target => _target;

  @override
  void set target(Expression expression) {
    _target = _becomeParentOf(expression as ExpressionImpl);
  }

  /**
   * If the AST structure has been resolved, and the function being invoked is
   * known based on static type information, then return the parameter element
   * representing the parameter to which the value of the index expression will
   * be bound. Otherwise, return `null`.
   */
  ParameterElement get _staticParameterElementForIndex {
    if (staticElement == null) {
      return null;
    }
    List<ParameterElement> parameters = staticElement.parameters;
    if (parameters.length < 1) {
      return null;
    }
    return parameters[0];
  }

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitIndexExpression(this);

  @override
  bool inGetterContext() {
    // TODO(brianwilkerson) Convert this to a getter.
    AstNode parent = this.parent;
    if (parent is AssignmentExpression) {
      AssignmentExpression assignment = parent;
      if (identical(assignment.leftHandSide, this) &&
          assignment.operator.type == TokenType.EQ) {
        return false;
      }
    }
    return true;
  }

  @override
  bool inSetterContext() {
    // TODO(brianwilkerson) Convert this to a getter.
    AstNode parent = this.parent;
    if (parent is PrefixExpression) {
      return parent.operator.type.isIncrementOperator;
    } else if (parent is PostfixExpression) {
      return true;
    } else if (parent is AssignmentExpression) {
      return identical(parent.leftHandSide, this);
    }
    return false;
  }

  @override
  void visitChildren(AstVisitor visitor) {
    _target?.accept(visitor);
    _index?.accept(visitor);
  }
}

/**
 * An instance creation expression.
 *
 *    newExpression ::=
 *        ('new' | 'const')? [TypeName] ('.' [SimpleIdentifier])? [ArgumentList]
 */
class InstanceCreationExpressionImpl extends ExpressionImpl
    implements InstanceCreationExpression {
  // TODO(brianwilkerson) Consider making InstanceCreationExpressionImpl extend
  // InvocationExpressionImpl. This would probably be a breaking change, but is
  // also probably worth it.

  /**
   * The 'new' or 'const' keyword used to indicate how an object should be
   * created, or `null` if the keyword is implicit.
   */
  @override
  Token keyword;

  /**
   * The name of the constructor to be invoked.
   */
  ConstructorNameImpl _constructorName;

  /**
   * The type arguments associated with the constructor, rather than with the
   * class in which the constructor is defined. It is always an error if there
   * are type arguments because Dart doesn't currently support generic
   * constructors, but we capture them in the AST in order to recover better.
   */
  TypeArgumentListImpl _typeArguments;

  /**
   * The list of arguments to the constructor.
   */
  ArgumentListImpl _argumentList;

  /**
   * The element associated with the constructor based on static type
   * information, or `null` if the AST structure has not been resolved or if the
   * constructor could not be resolved.
   */
  @override
  ConstructorElement staticElement;

  /**
   * Initialize a newly created instance creation expression.
   */
  InstanceCreationExpressionImpl(this.keyword,
      ConstructorNameImpl constructorName, ArgumentListImpl argumentList,
      {TypeArgumentListImpl typeArguments}) {
    _constructorName = _becomeParentOf(constructorName);
    _typeArguments = _becomeParentOf(typeArguments);
    _argumentList = _becomeParentOf(argumentList);
  }

  @override
  ArgumentList get argumentList => _argumentList;

  @override
  void set argumentList(ArgumentList argumentList) {
    _argumentList = _becomeParentOf(argumentList as ArgumentListImpl);
  }

  @override
  Token get beginToken => keyword ?? _constructorName.beginToken;

  @override
  Iterable<SyntacticEntity> get childEntities => new ChildEntities()
    ..add(keyword)
    ..add(_constructorName)
    ..add(_typeArguments)
    ..add(_argumentList);

  @override
  ConstructorName get constructorName => _constructorName;

  @override
  void set constructorName(ConstructorName name) {
    _constructorName = _becomeParentOf(name as ConstructorNameImpl);
  }

  @override
  Token get endToken => _argumentList.endToken;

  @override
  bool get isConst {
    if (!isImplicit) {
      return keyword.keyword == Keyword.CONST;
    } else {
      return inConstantContext;
    }
  }

  /**
   * Return `true` if this is an implicit constructor invocations.
   */
  bool get isImplicit => keyword == null;

  @override
  int get precedence => 16;

  /**
   * Return the type arguments associated with the constructor, rather than with
   * the class in which the constructor is defined. It is always an error if
   * there are type arguments because Dart doesn't currently support generic
   * constructors, but we capture them in the AST in order to recover better.
   */
  TypeArgumentList get typeArguments => _typeArguments;

  /**
   * Return the type arguments associated with the constructor, rather than with
   * the class in which the constructor is defined. It is always an error if
   * there are type arguments because Dart doesn't currently support generic
   * constructors, but we capture them in the AST in order to recover better.
   */
  void set typeArguments(TypeArgumentList typeArguments) {
    _typeArguments = _becomeParentOf(typeArguments as TypeArgumentListImpl);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) =>
      visitor.visitInstanceCreationExpression(this);

  /**
   * Return `true` if it would be valid for this instance creation expression to
   * have a keyword of `const`. It is valid if
   *
   * * the invoked constructor is a `const` constructor,
   * * all of the arguments are, or could be, constant expressions, and
   * * the evaluation of the constructor would not produce an exception.
   *
   * Note that this method will return `false` if the AST has not been resolved
   * because without resolution it cannot be determined whether the constructor
   * is a `const` constructor.
   *
   * Also note that this method can cause constant evaluation to occur, which
   * can be computationally expensive.
   * 
   * Deprecated: Use `LinterContext.canBeConst` instead.
   */
  @deprecated
  bool canBeConst() {
    //
    // Verify that the invoked constructor is a const constructor.
    //
    ConstructorElement element = staticElement;
    if (element == null || !element.isConst) {
      return false;
    }
    //
    // Verify that all of the arguments are, or could be, constant expressions.
    //
    for (Expression argument in argumentList.arguments) {
      argument = argument.unParenthesized;
      if (argument is NamedExpression) {
        argument = (argument as NamedExpression).expression.unParenthesized;
      }
      if (argument is InstanceCreationExpression) {
        if (!argument.isConst) {
          return false;
        }
      } else if (argument is TypedLiteral) {
        if (!argument.isConst) {
          return false;
        }
      } else if (argument is LiteralImpl) {
        if (argument is StringInterpolation) {
          return false;
        } else if (argument is AdjacentStrings) {
          for (StringLiteral string in (argument as AdjacentStrings).strings) {
            if (string is StringInterpolation) {
              return false;
            }
          }
        }
      } else if (argument is Identifier) {
        Element element = argument.staticElement;
        if (element is PropertyAccessorElement && !element.variable.isConst) {
          return false;
        } else if (element is VariableElement && !element.isConst) {
          return false;
        }
      } else {
        return false;
      }
    }
    //
    // Verify that the evaluation of the constructor would not produce an
    // exception.
    //
    Token oldKeyword = keyword;
    ConstantAnalysisErrorListener listener =
        new ConstantAnalysisErrorListener();
    try {
      keyword = new KeywordToken(Keyword.CONST, offset);
      LibraryElement library = element.library;
      AnalysisContext context = library.context;
      ErrorReporter errorReporter = new ErrorReporter(listener, element.source);
      accept(new ConstantVerifier(errorReporter, library, context.typeProvider,
          context.declaredVariables));
    } finally {
      keyword = oldKeyword;
    }
    return !listener.hasConstError;
  }

  @override
  void visitChildren(AstVisitor visitor) {
    _constructorName?.accept(visitor);
    _typeArguments?.accept(visitor);
    _argumentList?.accept(visitor);
  }
}

/**
 * An integer literal expression.
 *
 *    integerLiteral ::=
 *        decimalIntegerLiteral
 *      | hexadecimalIntegerLiteral
 *
 *    decimalIntegerLiteral ::=
 *        decimalDigit+
 *
 *    hexadecimalIntegerLiteral ::=
 *        '0x' hexadecimalDigit+
 *      | '0X' hexadecimalDigit+
 */
class IntegerLiteralImpl extends LiteralImpl implements IntegerLiteral {
  /**
   * The token representing the literal.
   */
  @override
  Token literal;

  /**
   * The value of the literal.
   */
  @override
  int value = 0;

  /**
   * Initialize a newly created integer literal.
   */
  IntegerLiteralImpl(this.literal, this.value);

  @override
  Token get beginToken => literal;

  @override
  Iterable<SyntacticEntity> get childEntities =>
      new ChildEntities()..add(literal);

  @override
  Token get endToken => literal;

  /**
   * Returns whether this literal's [parent] is a [PrefixExpression] of unary
   * negation.
   *
   * Note: this does *not* indicate that the value itself is negated, just that
   * the literal is the child of a negation operation. The literal value itself
   * will always be positive.
   */
  bool get immediatelyNegated {
    AstNode parent = this.parent; // Capture for type propagation.
    return parent is PrefixExpression &&
        parent.operator.type == TokenType.MINUS;
  }

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitIntegerLiteral(this);

  @override
  void visitChildren(AstVisitor visitor) {
    // There are no children to visit.
  }

  static bool isValidAsDouble(String lexeme) {
    // Less than 16 characters must be a valid double since it will be less than
    // 9007199254740992, 0x10000000000000, both 16 characters and 53 bits.
    if (lexeme.length < 16) {
      return true;
    }

    BigInt fullPrecision = BigInt.tryParse(lexeme);
    if (fullPrecision == null) {
      return false;
    }

    // Usually handled by the length check, however, we must check this before
    // constructing a mask later, or we'd get a negative-shift runtime error.
    int bitLengthAsInt = fullPrecision.bitLength;
    if (bitLengthAsInt <= 53) {
      return true;
    }

    // This would overflow the exponent (larger than maximum double).
    if (fullPrecision > BigInt.from(double.maxFinite)) {
      return false;
    }

    // Say [lexeme] uses 100 bits as an integer. The bottom 47 must be 0s -- so
    // construct a mask of 47 ones, via of 2^n - 1 where n is 47.
    BigInt bottomMask = (BigInt.one << (bitLengthAsInt - 53)) - BigInt.one;

    return fullPrecision & bottomMask == BigInt.zero;
  }

  /**
   * Return `true` if the given [lexeme] is a valid lexeme for an integer
   * literal. The flag [isNegative] should be `true` if the lexeme is preceded
   * by a unary negation operator.
   */
  static bool isValidAsInteger(String lexeme, bool isNegative) {
    // TODO(jmesserly): this depends on the platform int implementation, and
    // may not be accurate if run on dart4web.
    //
    // (Prior to https://dart-review.googlesource.com/c/sdk/+/63023 there was
    // a partial implementation here which may be a good starting point.
    // _isValidDecimalLiteral relied on int.parse so that would need some fixes.
    // _isValidHexadecimalLiteral worked except for negative int64 max.)
    if (isNegative) lexeme = '-$lexeme';
    return int.tryParse(lexeme) != null;
  }

  /**
   * Suggest the nearest valid double to a user. If the integer they wrote
   * requires more than a 53 bit mantissa, or more than 10 exponent bits, do
   * them the favor of suggesting the nearest integer that would work for them.
   */
  static double nearestValidDouble(String lexeme) =>
      math.min(double.maxFinite, BigInt.parse(lexeme).toDouble());
}

/**
 * A node within a [StringInterpolation].
 *
 *    interpolationElement ::=
 *        [InterpolationExpression]
 *      | [InterpolationString]
 */
abstract class InterpolationElementImpl extends AstNodeImpl
    implements InterpolationElement {}

/**
 * An expression embedded in a string interpolation.
 *
 *    interpolationExpression ::=
 *        '$' [SimpleIdentifier]
 *      | '$' '{' [Expression] '}'
 */
class InterpolationExpressionImpl extends InterpolationElementImpl
    implements InterpolationExpression {
  /**
   * The token used to introduce the interpolation expression; either '$' if the
   * expression is a simple identifier or '${' if the expression is a full
   * expression.
   */
  @override
  Token leftBracket;

  /**
   * The expression to be evaluated for the value to be converted into a string.
   */
  ExpressionImpl _expression;

  /**
   * The right curly bracket, or `null` if the expression is an identifier
   * without brackets.
   */
  @override
  Token rightBracket;

  /**
   * Initialize a newly created interpolation expression.
   */
  InterpolationExpressionImpl(
      this.leftBracket, ExpressionImpl expression, this.rightBracket) {
    _expression = _becomeParentOf(expression);
  }

  @override
  Token get beginToken => leftBracket;

  @override
  Iterable<SyntacticEntity> get childEntities => new ChildEntities()
    ..add(leftBracket)
    ..add(_expression)
    ..add(rightBracket);

  @override
  Token get endToken {
    if (rightBracket != null) {
      return rightBracket;
    }
    return _expression.endToken;
  }

  @override
  Expression get expression => _expression;

  @override
  void set expression(Expression expression) {
    _expression = _becomeParentOf(expression as ExpressionImpl);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) =>
      visitor.visitInterpolationExpression(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _expression?.accept(visitor);
  }
}

/**
 * A non-empty substring of an interpolated string.
 *
 *    interpolationString ::=
 *        characters
 */
class InterpolationStringImpl extends InterpolationElementImpl
    implements InterpolationString {
  /**
   * The characters that will be added to the string.
   */
  @override
  Token contents;

  /**
   * The value of the literal.
   */
  @override
  String value;

  /**
   * Initialize a newly created string of characters that are part of a string
   * interpolation.
   */
  InterpolationStringImpl(this.contents, this.value);

  @override
  Token get beginToken => contents;

  @override
  Iterable<SyntacticEntity> get childEntities =>
      new ChildEntities()..add(contents);

  @override
  int get contentsEnd {
    String lexeme = contents.lexeme;
    return offset + new StringLexemeHelper(lexeme, true, true).end;
  }

  @override
  int get contentsOffset {
    int offset = contents.offset;
    String lexeme = contents.lexeme;
    return offset + new StringLexemeHelper(lexeme, true, true).start;
  }

  @override
  Token get endToken => contents;

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitInterpolationString(this);

  @override
  void visitChildren(AstVisitor visitor) {}
}

/**
 * Common base class for [FunctionExpressionInvocationImpl] and
 * [MethodInvocationImpl].
 */
abstract class InvocationExpressionImpl extends ExpressionImpl
    implements InvocationExpression {
  /**
   * The list of arguments to the function.
   */
  ArgumentListImpl _argumentList;

  /**
   * The type arguments to be applied to the method being invoked, or `null` if
   * no type arguments were provided.
   */
  TypeArgumentListImpl _typeArguments;

  @override
  DartType staticInvokeType;

  /**
   * Initialize a newly created invocation.
   */
  InvocationExpressionImpl(
      TypeArgumentListImpl typeArguments, ArgumentListImpl argumentList) {
    _typeArguments = _becomeParentOf(typeArguments);
    _argumentList = _becomeParentOf(argumentList);
  }

  @override
  ArgumentList get argumentList => _argumentList;

  void set argumentList(ArgumentList argumentList) {
    _argumentList = _becomeParentOf(argumentList as ArgumentListImpl);
  }

  @deprecated
  @override
  DartType get propagatedInvokeType => null;

  @deprecated
  @override
  set propagatedInvokeType(DartType type) {}

  @override
  TypeArgumentList get typeArguments => _typeArguments;

  void set typeArguments(TypeArgumentList typeArguments) {
    _typeArguments = _becomeParentOf(typeArguments as TypeArgumentListImpl);
  }
}

/**
 * An is expression.
 *
 *    isExpression ::=
 *        [Expression] 'is' '!'? [TypeName]
 */
class IsExpressionImpl extends ExpressionImpl implements IsExpression {
  /**
   * The expression used to compute the value whose type is being tested.
   */
  ExpressionImpl _expression;

  /**
   * The is operator.
   */
  @override
  Token isOperator;

  /**
   * The not operator, or `null` if the sense of the test is not negated.
   */
  @override
  Token notOperator;

  /**
   * The name of the type being tested for.
   */
  TypeAnnotationImpl _type;

  /**
   * Initialize a newly created is expression. The [notOperator] can be `null`
   * if the sense of the test is not negated.
   */
  IsExpressionImpl(ExpressionImpl expression, this.isOperator, this.notOperator,
      TypeAnnotationImpl type) {
    _expression = _becomeParentOf(expression);
    _type = _becomeParentOf(type);
  }

  @override
  Token get beginToken => _expression.beginToken;

  @override
  Iterable<SyntacticEntity> get childEntities => new ChildEntities()
    ..add(_expression)
    ..add(isOperator)
    ..add(notOperator)
    ..add(_type);

  @override
  Token get endToken => _type.endToken;

  @override
  Expression get expression => _expression;

  @override
  void set expression(Expression expression) {
    _expression = _becomeParentOf(expression as ExpressionImpl);
  }

  @override
  int get precedence => 7;

  @override
  TypeAnnotation get type => _type;

  @override
  void set type(TypeAnnotation type) {
    _type = _becomeParentOf(type as TypeAnnotationImpl);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitIsExpression(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _expression?.accept(visitor);
    _type?.accept(visitor);
  }
}

/**
 * A statement that has a label associated with them.
 *
 *    labeledStatement ::=
 *       [Label]+ [Statement]
 */
class LabeledStatementImpl extends StatementImpl implements LabeledStatement {
  /**
   * The labels being associated with the statement.
   */
  NodeList<Label> _labels;

  /**
   * The statement with which the labels are being associated.
   */
  StatementImpl _statement;

  /**
   * Initialize a newly created labeled statement.
   */
  LabeledStatementImpl(List<Label> labels, StatementImpl statement) {
    _labels = new NodeListImpl<Label>(this, labels);
    _statement = _becomeParentOf(statement);
  }

  @override
  Token get beginToken {
    if (!_labels.isEmpty) {
      return _labels.beginToken;
    }
    return _statement.beginToken;
  }

  @override
  Iterable<SyntacticEntity> get childEntities => new ChildEntities()
    ..addAll(_labels)
    ..add(_statement);

  @override
  Token get endToken => _statement.endToken;

  @override
  NodeList<Label> get labels => _labels;

  @override
  Statement get statement => _statement;

  @override
  void set statement(Statement statement) {
    _statement = _becomeParentOf(statement as StatementImpl);
  }

  @override
  Statement get unlabeled => _statement.unlabeled;

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitLabeledStatement(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _labels.accept(visitor);
    _statement?.accept(visitor);
  }
}

/**
 * A label on either a [LabeledStatement] or a [NamedExpression].
 *
 *    label ::=
 *        [SimpleIdentifier] ':'
 */
class LabelImpl extends AstNodeImpl implements Label {
  /**
   * The label being associated with the statement.
   */
  SimpleIdentifierImpl _label;

  /**
   * The colon that separates the label from the statement.
   */
  @override
  Token colon;

  /**
   * Initialize a newly created label.
   */
  LabelImpl(SimpleIdentifierImpl label, this.colon) {
    _label = _becomeParentOf(label);
  }

  @override
  Token get beginToken => _label.beginToken;

  @override
  Iterable<SyntacticEntity> get childEntities =>
      new ChildEntities()..add(_label)..add(colon);

  @override
  Token get endToken => colon;

  @override
  SimpleIdentifier get label => _label;

  @override
  void set label(SimpleIdentifier label) {
    _label = _becomeParentOf(label as SimpleIdentifierImpl);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitLabel(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _label?.accept(visitor);
  }
}

/**
 * A library directive.
 *
 *    libraryDirective ::=
 *        [Annotation] 'library' [Identifier] ';'
 */
class LibraryDirectiveImpl extends DirectiveImpl implements LibraryDirective {
  /**
   * The token representing the 'library' keyword.
   */
  @override
  Token libraryKeyword;

  /**
   * The name of the library being defined.
   */
  LibraryIdentifierImpl _name;

  /**
   * The semicolon terminating the directive.
   */
  @override
  Token semicolon;

  /**
   * Initialize a newly created library directive. Either or both of the
   * [comment] and [metadata] can be `null` if the directive does not have the
   * corresponding attribute.
   */
  LibraryDirectiveImpl(CommentImpl comment, List<Annotation> metadata,
      this.libraryKeyword, LibraryIdentifierImpl name, this.semicolon)
      : super(comment, metadata) {
    _name = _becomeParentOf(name);
  }

  @override
  Iterable<SyntacticEntity> get childEntities =>
      super._childEntities..add(libraryKeyword)..add(_name)..add(semicolon);

  @override
  Token get endToken => semicolon;

  @override
  Token get firstTokenAfterCommentAndMetadata => libraryKeyword;

  @override
  Token get keyword => libraryKeyword;

  @override
  LibraryIdentifier get name => _name;

  @override
  void set name(LibraryIdentifier name) {
    _name = _becomeParentOf(name as LibraryIdentifierImpl);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitLibraryDirective(this);

  @override
  void visitChildren(AstVisitor visitor) {
    super.visitChildren(visitor);
    _name?.accept(visitor);
  }
}

/**
 * The identifier for a library.
 *
 *    libraryIdentifier ::=
 *        [SimpleIdentifier] ('.' [SimpleIdentifier])*
 */
class LibraryIdentifierImpl extends IdentifierImpl
    implements LibraryIdentifier {
  /**
   * The components of the identifier.
   */
  NodeList<SimpleIdentifier> _components;

  /**
   * Initialize a newly created prefixed identifier.
   */
  LibraryIdentifierImpl(List<SimpleIdentifier> components) {
    _components = new NodeListImpl<SimpleIdentifier>(this, components);
  }

  @override
  Token get beginToken => _components.beginToken;

  @override
  @deprecated
  Element get bestElement => staticElement;

  @override
  // TODO(paulberry): add "." tokens.
  Iterable<SyntacticEntity> get childEntities =>
      new ChildEntities()..addAll(_components);

  @override
  NodeList<SimpleIdentifier> get components => _components;

  @override
  Token get endToken => _components.endToken;

  @override
  String get name {
    StringBuffer buffer = new StringBuffer();
    bool needsPeriod = false;
    int length = _components.length;
    for (int i = 0; i < length; i++) {
      SimpleIdentifier identifier = _components[i];
      if (needsPeriod) {
        buffer.write(".");
      } else {
        needsPeriod = true;
      }
      buffer.write(identifier.name);
    }
    return buffer.toString();
  }

  @override
  int get precedence => 15;

  @deprecated
  @override
  Element get propagatedElement => null;

  @override
  Element get staticElement => null;

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitLibraryIdentifier(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _components.accept(visitor);
  }
}

/**
 * A list literal.
 *
 *    listLiteral ::=
 *        'const'? ('<' [TypeAnnotation] '>')?
 *        '[' ([CollectionLiteralElement] ','?)? ']'
 *
 * This is the class that is used to represent a list literal when either the
 * 'control-flow-collections' or 'spread-collections' experiments are enabled.
 * If neither of those experiments are enabled, then [ListLiteral] will be used.
 */
class ListLiteral2Impl extends TypedLiteralImpl implements ListLiteral2 {
  @override
  Token leftBracket;

  /**
   * The elements used to compute the elements of the list.
   */
  NodeList<CollectionElement> _elements;

  @override
  Token rightBracket;

  /**
   * Initialize a newly created list literal. The [constKeyword] can be `null`
   * if the literal is not a constant. The [typeArguments] can be `null` if no
   * type arguments were declared. The list of [elements] can be `null` if the
   * list is empty.
   */
  ListLiteral2Impl(Token constKeyword, TypeArgumentListImpl typeArguments,
      this.leftBracket, List<CollectionElement> elements, this.rightBracket)
      : super(constKeyword, typeArguments) {
    _elements = new NodeListImpl<CollectionElement>(this, elements);
  }

  @override
  Token get beginToken {
    if (constKeyword != null) {
      return constKeyword;
    }
    TypeArgumentList typeArguments = this.typeArguments;
    if (typeArguments != null) {
      return typeArguments.beginToken;
    }
    return leftBracket;
  }

  @override
  // TODO(paulberry): add commas.
  Iterable<SyntacticEntity> get childEntities => super._childEntities
    ..add(leftBracket)
    ..addAll(_elements)
    ..add(rightBracket);

  @override
  NodeList<CollectionElement> get elements => _elements;

  @override
  Token get endToken => rightBracket;

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitListLiteral2(this);

  @override
  void visitChildren(AstVisitor visitor) {
    super.visitChildren(visitor);
    _elements.accept(visitor);
  }
}

/**
 * A list literal.
 *
 *    listLiteral ::=
 *        'const'? ('<' [TypeName] '>')? '[' ([Expression] ','?)? ']'
 *
 * This is the class that is used to represent a list literal when neither the
 * 'control-flow-collections' nor 'spread-collections' experiments are enabled.
 * If either of those experiments are enabled, then [ListLiteral2] will be used.
 */
class ListLiteralImpl extends TypedLiteralImpl implements ListLiteral {
  /**
   * The left square bracket.
   */
  @override
  Token leftBracket;

  /**
   * The expressions used to compute the elements of the list.
   */
  NodeList<Expression> _elements;

  /**
   * The right square bracket.
   */
  @override
  Token rightBracket;

  /**
   * Initialize a newly created list literal. The [constKeyword] can be `null`
   * if the literal is not a constant. The [typeArguments] can be `null` if no
   * type arguments were declared. The list of [elements] can be `null` if the
   * list is empty.
   */
  ListLiteralImpl(Token constKeyword, TypeArgumentListImpl typeArguments,
      this.leftBracket, List<Expression> elements, this.rightBracket)
      : super(constKeyword, typeArguments) {
    _elements = new NodeListImpl<Expression>(this, elements);
  }

  @override
  Token get beginToken {
    if (constKeyword != null) {
      return constKeyword;
    }
    TypeArgumentList typeArguments = this.typeArguments;
    if (typeArguments != null) {
      return typeArguments.beginToken;
    }
    return leftBracket;
  }

  @override
  // TODO(paulberry): add commas.
  Iterable<SyntacticEntity> get childEntities => super._childEntities
    ..add(leftBracket)
    ..addAll(_elements)
    ..add(rightBracket);

  @override
  NodeList<Expression> get elements => _elements;

  @override
  Token get endToken => rightBracket;

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitListLiteral(this);

  @override
  void visitChildren(AstVisitor visitor) {
    super.visitChildren(visitor);
    _elements.accept(visitor);
  }
}

/**
 * A node that represents a literal expression.
 *
 *    literal ::=
 *        [BooleanLiteral]
 *      | [DoubleLiteral]
 *      | [IntegerLiteral]
 *      | [ListLiteral]
 *      | [MapLiteral]
 *      | [NullLiteral]
 *      | [StringLiteral]
 */
abstract class LiteralImpl extends ExpressionImpl implements Literal {
  @override
  int get precedence => 16;
}

/**
 * Additional information about local variables within a function or method
 * produced at resolution time.
 */
class LocalVariableInfo {
  /**
   * The set of local variables and parameters that are potentially mutated
   * within a local function other than the function in which they are declared.
   */
  final Set<VariableElement> potentiallyMutatedInClosure =
      new Set<VariableElement>();

  /**
   * The set of local variables and parameters that are potentially mutated
   * within the scope of their declarations.
   */
  final Set<VariableElement> potentiallyMutatedInScope =
      new Set<VariableElement>();
}

abstract class MapElementImpl extends AstNodeImpl implements MapElement {}

class MapForElementImpl extends MapElementImpl
    with ForMixin
    implements MapForElement {
  /**
   * The body of the loop.
   */
  MapElementImpl _body;

  /**
   * Initialize a newly created for element.
   */
  MapForElementImpl(
      Token awaitKeyword,
      Token forKeyword,
      Token leftParenthesis,
      ForLoopPartsImpl forLoopParts,
      Token rightParenthesis,
      MapElementImpl body) {
    this.awaitKeyword = awaitKeyword;
    this.forKeyword = forKeyword;
    this.leftParenthesis = leftParenthesis;
    _forLoopParts = _becomeParentOf(forLoopParts);
    this.rightParenthesis = rightParenthesis;
    _body = _becomeParentOf(body);
  }

  @override
  MapElement get body => _body;

  void set body(MapElement statement) {
    _body = _becomeParentOf(statement as MapElementImpl);
  }

  @override
  Iterable<SyntacticEntity> get childEntities => new ChildEntities()
    ..addAll(super.childEntities)
    ..add(_body);

  @override
  Token get endToken => _body.endToken;

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitMapForElement(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _forLoopParts?.accept(visitor);
    _body?.accept(visitor);
  }
}

class MapIfElementImpl extends MapElementImpl
    with IfMixin
    implements MapIfElement {
  /**
   * The element to be executed if the condition is `true`.
   */
  MapElementImpl _thenElement;

  /**
   * The element to be executed if the condition is `false`, or `null` if there
   * is no such element.
   */
  MapElementImpl _elseElement;

  /**
   * Initialize a newly created for element.
   */
  MapIfElementImpl(
      Token ifKeyword,
      Token leftParenthesis,
      ExpressionImpl condition,
      Token rightParenthesis,
      MapElementImpl thenElement,
      Token elseKeyword,
      MapElementImpl elseElement) {
    this.ifKeyword = ifKeyword;
    this.leftParenthesis = leftParenthesis;
    _condition = _becomeParentOf(condition);
    this.rightParenthesis = rightParenthesis;
    _thenElement = _becomeParentOf(thenElement);
    this.elseKeyword = elseKeyword;
    _elseElement = _becomeParentOf(elseElement);
  }

  @override
  Iterable<SyntacticEntity> get childEntities => new ChildEntities()
    ..addAll(super.childEntities)
    ..add(_thenElement)
    ..add(_elseElement);

  @override
  MapElement get elseElement => _elseElement;

  set elseElement(MapElement element) {
    _elseElement = _becomeParentOf(element as MapElementImpl);
  }

  @override
  Token get endToken => _elseElement?.endToken ?? _thenElement.endToken;

  @override
  MapElement get thenElement => _thenElement;

  set thenElement(MapElement element) {
    _thenElement = _becomeParentOf(element as MapElementImpl);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitMapIfElement(this);

  @override
  void visitChildren(AstVisitor visitor) {
    super.visitChildren(visitor);
    _thenElement?.accept(visitor);
    _elseElement?.accept(visitor);
  }
}

/**
 * A literal map.
 *
 *    mapLiteral ::=
 *        'const'? ('<' [TypeAnnotation] (',' [TypeAnnotation])* '>')?
 *        '{' ([MapElement] (',' [MapElement])* ','?)? '}'
 *
 * This is the class that is used to represent a map literal when either the
 * 'control-flow-collections' or 'spread-collections' experiments are enabled.
 * If neither of those experiments are enabled, then [MapLiteral] will be used.
 */
class MapLiteral2Impl extends TypedLiteralImpl implements MapLiteral2 {
  @override
  Token leftBracket;

  /**
   * The entries in the map.
   */
  NodeList<MapElement> _entries;

  @override
  Token rightBracket;

  /**
   * Initialize a newly created map literal. The [constKeyword] can be `null` if
   * the literal is not a constant. The [typeArguments] can be `null` if no type
   * arguments were declared. The [entries] can be `null` if the map is empty.
   */
  MapLiteral2Impl(Token constKeyword, TypeArgumentListImpl typeArguments,
      this.leftBracket, List<MapElement> entries, this.rightBracket)
      : super(constKeyword, typeArguments) {
    _entries = new NodeListImpl<MapElement>(this, entries);
  }

  @override
  Token get beginToken {
    if (constKeyword != null) {
      return constKeyword;
    }
    TypeArgumentList typeArguments = this.typeArguments;
    if (typeArguments != null) {
      return typeArguments.beginToken;
    }
    return leftBracket;
  }

  @override
  // TODO(paulberry): add commas.
  Iterable<SyntacticEntity> get childEntities => super._childEntities
    ..add(leftBracket)
    ..addAll(entries)
    ..add(rightBracket);

  @override
  Token get endToken => rightBracket;

  @override
  NodeList<MapElement> get entries => _entries;

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitMapLiteral2(this);

  @override
  void visitChildren(AstVisitor visitor) {
    super.visitChildren(visitor);
    _entries.accept(visitor);
  }
}

/**
 * A single key/value pair in a map literal.
 *
 *    mapLiteralEntry ::=
 *        [Expression] ':' [Expression]
 */
class MapLiteralEntryImpl extends MapElementImpl implements MapLiteralEntry {
  /**
   * The expression computing the key with which the value will be associated.
   */
  ExpressionImpl _key;

  /**
   * The colon that separates the key from the value.
   */
  @override
  Token separator;

  /**
   * The expression computing the value that will be associated with the key.
   */
  ExpressionImpl _value;

  /**
   * Initialize a newly created map literal entry.
   */
  MapLiteralEntryImpl(
      ExpressionImpl key, this.separator, ExpressionImpl value) {
    _key = _becomeParentOf(key);
    _value = _becomeParentOf(value);
  }

  @override
  Token get beginToken => _key.beginToken;

  @override
  Iterable<SyntacticEntity> get childEntities =>
      new ChildEntities()..add(_key)..add(separator)..add(_value);

  @override
  Token get endToken => _value.endToken;

  @override
  Expression get key => _key;

  @override
  void set key(Expression string) {
    _key = _becomeParentOf(string as ExpressionImpl);
  }

  @override
  Expression get value => _value;

  @override
  void set value(Expression expression) {
    _value = _becomeParentOf(expression as ExpressionImpl);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitMapLiteralEntry(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _key?.accept(visitor);
    _value?.accept(visitor);
  }
}

/**
 * A literal map.
 *
 *    mapLiteral ::=
 *        'const'? ('<' [TypeName] (',' [TypeName])* '>')?
 *        '{' ([MapLiteralEntry] (',' [MapLiteralEntry])* ','?)? '}'
 */
class MapLiteralImpl extends TypedLiteralImpl implements MapLiteral {
  /**
   * The left curly bracket.
   */
  @override
  Token leftBracket;

  /**
   * The entries in the map.
   */
  NodeList<MapLiteralEntry> _entries;

  /**
   * The right curly bracket.
   */
  @override
  Token rightBracket;

  /**
   * Initialize a newly created map literal. The [constKeyword] can be `null` if
   * the literal is not a constant. The [typeArguments] can be `null` if no type
   * arguments were declared. The [entries] can be `null` if the map is empty.
   */
  MapLiteralImpl(Token constKeyword, TypeArgumentListImpl typeArguments,
      this.leftBracket, List<MapLiteralEntry> entries, this.rightBracket)
      : super(constKeyword, typeArguments) {
    _entries = new NodeListImpl<MapLiteralEntry>(this, entries);
  }

  @override
  Token get beginToken {
    if (constKeyword != null) {
      return constKeyword;
    }
    TypeArgumentList typeArguments = this.typeArguments;
    if (typeArguments != null) {
      return typeArguments.beginToken;
    }
    return leftBracket;
  }

  @override
  // TODO(paulberry): add commas.
  Iterable<SyntacticEntity> get childEntities => super._childEntities
    ..add(leftBracket)
    ..addAll(entries)
    ..add(rightBracket);

  @override
  Token get endToken => rightBracket;

  @override
  NodeList<MapLiteralEntry> get entries => _entries;

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitMapLiteral(this);

  @override
  void visitChildren(AstVisitor visitor) {
    super.visitChildren(visitor);
    _entries.accept(visitor);
  }
}

/**
 * A method declaration.
 *
 *    methodDeclaration ::=
 *        methodSignature [FunctionBody]
 *
 *    methodSignature ::=
 *        'external'? ('abstract' | 'static')? [Type]? ('get' | 'set')?
 *        methodName [TypeParameterList] [FormalParameterList]
 *
 *    methodName ::=
 *        [SimpleIdentifier]
 *      | 'operator' [SimpleIdentifier]
 */
class MethodDeclarationImpl extends ClassMemberImpl
    implements MethodDeclaration {
  /**
   * The token for the 'external' keyword, or `null` if the constructor is not
   * external.
   */
  @override
  Token externalKeyword;

  /**
   * The token representing the 'abstract' or 'static' keyword, or `null` if
   * neither modifier was specified.
   */
  @override
  Token modifierKeyword;

  /**
   * The return type of the method, or `null` if no return type was declared.
   */
  TypeAnnotationImpl _returnType;

  /**
   * The token representing the 'get' or 'set' keyword, or `null` if this is a
   * method declaration rather than a property declaration.
   */
  @override
  Token propertyKeyword;

  /**
   * The token representing the 'operator' keyword, or `null` if this method
   * does not declare an operator.
   */
  @override
  Token operatorKeyword;

  /**
   * The name of the method.
   */
  SimpleIdentifierImpl _name;

  /**
   * The type parameters associated with the method, or `null` if the method is
   * not a generic method.
   */
  TypeParameterListImpl _typeParameters;

  /**
   * The parameters associated with the method, or `null` if this method
   * declares a getter.
   */
  FormalParameterListImpl _parameters;

  /**
   * The body of the method.
   */
  FunctionBodyImpl _body;

  /**
   * Initialize a newly created method declaration. Either or both of the
   * [comment] and [metadata] can be `null` if the declaration does not have the
   * corresponding attribute. The [externalKeyword] can be `null` if the method
   * is not external. The [modifierKeyword] can be `null` if the method is
   * neither abstract nor static. The [returnType] can be `null` if no return
   * type was specified. The [propertyKeyword] can be `null` if the method is
   * neither a getter or a setter. The [operatorKeyword] can be `null` if the
   * method does not implement an operator. The [parameters] must be `null` if
   * this method declares a getter.
   */
  MethodDeclarationImpl(
      CommentImpl comment,
      List<Annotation> metadata,
      this.externalKeyword,
      this.modifierKeyword,
      TypeAnnotationImpl returnType,
      this.propertyKeyword,
      this.operatorKeyword,
      SimpleIdentifierImpl name,
      TypeParameterListImpl typeParameters,
      FormalParameterListImpl parameters,
      FunctionBodyImpl body)
      : super(comment, metadata) {
    _returnType = _becomeParentOf(returnType);
    _name = _becomeParentOf(name);
    _typeParameters = _becomeParentOf(typeParameters);
    _parameters = _becomeParentOf(parameters);
    _body = _becomeParentOf(body);
  }

  @override
  FunctionBody get body => _body;

  @override
  void set body(FunctionBody functionBody) {
    _body = _becomeParentOf(functionBody as FunctionBodyImpl);
  }

  @override
  Iterable<SyntacticEntity> get childEntities => super._childEntities
    ..add(externalKeyword)
    ..add(modifierKeyword)
    ..add(_returnType)
    ..add(propertyKeyword)
    ..add(operatorKeyword)
    ..add(_name)
    ..add(_parameters)
    ..add(_body);

  /**
   * Return the element associated with this method, or `null` if the AST
   * structure has not been resolved. The element can either be a
   * [MethodElement], if this represents the declaration of a normal method, or
   * a [PropertyAccessorElement] if this represents the declaration of either a
   * getter or a setter.
   */
  @override
  ExecutableElement get declaredElement =>
      _name?.staticElement as ExecutableElement;

  @deprecated
  @override
  ExecutableElement get element => declaredElement;

  @override
  Token get endToken => _body.endToken;

  @override
  Token get firstTokenAfterCommentAndMetadata {
    if (externalKeyword != null) {
      return externalKeyword;
    } else if (modifierKeyword != null) {
      return modifierKeyword;
    } else if (_returnType != null) {
      return _returnType.beginToken;
    } else if (propertyKeyword != null) {
      return propertyKeyword;
    } else if (operatorKeyword != null) {
      return operatorKeyword;
    }
    return _name.beginToken;
  }

  @override
  bool get isAbstract {
    FunctionBody body = _body;
    return externalKeyword == null &&
        (body is EmptyFunctionBody && !body.semicolon.isSynthetic);
  }

  @override
  bool get isGetter => propertyKeyword?.keyword == Keyword.GET;

  @override
  bool get isOperator => operatorKeyword != null;

  @override
  bool get isSetter => propertyKeyword?.keyword == Keyword.SET;

  @override
  bool get isStatic => modifierKeyword?.keyword == Keyword.STATIC;

  @override
  SimpleIdentifier get name => _name;

  @override
  void set name(SimpleIdentifier identifier) {
    _name = _becomeParentOf(identifier as SimpleIdentifierImpl);
  }

  @override
  FormalParameterList get parameters => _parameters;

  @override
  void set parameters(FormalParameterList parameters) {
    _parameters = _becomeParentOf(parameters as FormalParameterListImpl);
  }

  @override
  TypeAnnotation get returnType => _returnType;

  @override
  void set returnType(TypeAnnotation type) {
    _returnType = _becomeParentOf(type as TypeAnnotationImpl);
  }

  @override
  TypeParameterList get typeParameters => _typeParameters;

  @override
  void set typeParameters(TypeParameterList typeParameters) {
    _typeParameters = _becomeParentOf(typeParameters as TypeParameterListImpl);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitMethodDeclaration(this);

  @override
  void visitChildren(AstVisitor visitor) {
    super.visitChildren(visitor);
    _returnType?.accept(visitor);
    _name?.accept(visitor);
    _typeParameters?.accept(visitor);
    _parameters?.accept(visitor);
    _body?.accept(visitor);
  }
}

/**
 * The invocation of either a function or a method. Invocations of functions
 * resulting from evaluating an expression are represented by
 * [FunctionExpressionInvocation] nodes. Invocations of getters and setters are
 * represented by either [PrefixedIdentifier] or [PropertyAccess] nodes.
 *
 *    methodInvocation ::=
 *        ([Expression] '.')? [SimpleIdentifier] [TypeArgumentList]? [ArgumentList]
 */
class MethodInvocationImpl extends InvocationExpressionImpl
    implements MethodInvocation {
  /**
   * The expression producing the object on which the method is defined, or
   * `null` if there is no target (that is, the target is implicitly `this`).
   */
  ExpressionImpl _target;

  /**
   * The operator that separates the target from the method name, or `null`
   * if there is no target. In an ordinary method invocation this will be a
   * period ('.'). In a cascade section this will be the cascade operator
   * ('..').
   */
  @override
  Token operator;

  /**
   * The name of the method being invoked.
   */
  SimpleIdentifierImpl _methodName;

  /**
   * The invoke type of the [methodName] if the target element is a getter,
   * or `null` otherwise.
   */
  DartType _methodNameType;

  /**
   * Initialize a newly created method invocation. The [target] and [operator]
   * can be `null` if there is no target.
   */
  MethodInvocationImpl(
      ExpressionImpl target,
      this.operator,
      SimpleIdentifierImpl methodName,
      TypeArgumentListImpl typeArguments,
      ArgumentListImpl argumentList)
      : super(typeArguments, argumentList) {
    _target = _becomeParentOf(target);
    _methodName = _becomeParentOf(methodName);
  }

  @override
  Token get beginToken {
    if (_target != null) {
      return _target.beginToken;
    } else if (operator != null) {
      return operator;
    }
    return _methodName.beginToken;
  }

  @override
  Iterable<SyntacticEntity> get childEntities => new ChildEntities()
    ..add(_target)
    ..add(operator)
    ..add(_methodName)
    ..add(_argumentList);

  @override
  Token get endToken => _argumentList.endToken;

  @override
  Expression get function => methodName;

  @override
  bool get isCascaded =>
      operator != null && operator.type == TokenType.PERIOD_PERIOD;

  @override
  SimpleIdentifier get methodName => _methodName;

  @override
  void set methodName(SimpleIdentifier identifier) {
    _methodName = _becomeParentOf(identifier as SimpleIdentifierImpl);
  }

  /**
   * The invoke type of the [methodName].
   *
   * If the target element is a [MethodElement], this is the same as the
   * [staticInvokeType]. If the target element is a getter, presumably
   * returning an [ExecutableElement] so that it can be invoked in this
   * [MethodInvocation], then this type is the type of the getter, and the
   * [staticInvokeType] is the invoked type of the returned element.
   */
  DartType get methodNameType => _methodNameType ?? staticInvokeType;

  /**
   * Set the [methodName] invoke type, only if the target element is a getter.
   * Otherwise, the target element itself is invoked, [_methodNameType] is
   * `null`, and the getter will return [staticInvokeType].
   */
  set methodNameType(DartType methodNameType) {
    _methodNameType = methodNameType;
  }

  @override
  int get precedence => 15;

  @override
  Expression get realTarget {
    if (isCascaded) {
      AstNode ancestor = parent;
      while (ancestor is! CascadeExpression) {
        if (ancestor == null) {
          return _target;
        }
        ancestor = ancestor.parent;
      }
      return (ancestor as CascadeExpression).target;
    }
    return _target;
  }

  @override
  Expression get target => _target;

  @override
  void set target(Expression expression) {
    _target = _becomeParentOf(expression as ExpressionImpl);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitMethodInvocation(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _target?.accept(visitor);
    _methodName?.accept(visitor);
    _typeArguments?.accept(visitor);
    _argumentList?.accept(visitor);
  }
}

/**
 * The declaration of a mixin.
 *
 *    mixinDeclaration ::=
 *        metadata? 'mixin' [SimpleIdentifier] [TypeParameterList]?
 *        [RequiresClause]? [ImplementsClause]? '{' [ClassMember]* '}'
 */
class MixinDeclarationImpl extends ClassOrMixinDeclarationImpl
    implements MixinDeclaration {
  @override
  Token mixinKeyword;

  /**
   * The on clause for the mixin, or `null` if the mixin does not have any
   * super-class constraints.
   */
  OnClauseImpl _onClause;

  /**
   * Initialize a newly created mixin declaration. Either or both of the
   * [comment] and [metadata] can be `null` if the mixin does not have the
   * corresponding attribute. The [typeParameters] can be `null` if the mixin does not
   * have any type parameters. Either or both of the [onClause],
   * and [implementsClause] can be `null` if the mixin does not have the
   * corresponding clause. The list of [members] can be `null` if the mixin does
   * not have any members.
   */
  MixinDeclarationImpl(
      CommentImpl comment,
      List<Annotation> metadata,
      this.mixinKeyword,
      SimpleIdentifierImpl name,
      TypeParameterListImpl typeParameters,
      OnClauseImpl onClause,
      ImplementsClauseImpl implementsClause,
      Token leftBracket,
      List<ClassMember> members,
      Token rightBracket)
      : super(comment, metadata, name, typeParameters, implementsClause,
            leftBracket, members, rightBracket) {
    _onClause = _becomeParentOf(onClause);
  }

  @override
  Iterable<SyntacticEntity> get childEntities => super._childEntities
    ..add(mixinKeyword)
    ..add(_name)
    ..add(_typeParameters)
    ..add(_onClause)
    ..add(_implementsClause)
    ..add(leftBracket)
    ..addAll(members)
    ..add(rightBracket);

  @override
  ClassElement get declaredElement => _name?.staticElement as ClassElement;

  @deprecated
  @override
  Element get element => declaredElement;

  @override
  Token get firstTokenAfterCommentAndMetadata {
    return mixinKeyword;
  }

  @override
  ImplementsClause get implementsClause => _implementsClause;

  void set implementsClause(ImplementsClause implementsClause) {
    _implementsClause =
        _becomeParentOf(implementsClause as ImplementsClauseImpl);
  }

  @override
  NodeList<ClassMember> get members => _members;

  @override
  OnClause get onClause => _onClause;

  void set onClause(OnClause onClause) {
    _onClause = _becomeParentOf(onClause as OnClauseImpl);
  }

  @override
  TypeParameterList get typeParameters => _typeParameters;

  void set typeParameters(TypeParameterList typeParameters) {
    _typeParameters = _becomeParentOf(typeParameters as TypeParameterListImpl);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitMixinDeclaration(this);

  @override
  void visitChildren(AstVisitor visitor) {
    super.visitChildren(visitor);
    _name?.accept(visitor);
    _typeParameters?.accept(visitor);
    _onClause?.accept(visitor);
    _implementsClause?.accept(visitor);
    members.accept(visitor);
  }
}

/**
 * A node that declares a single name within the scope of a compilation unit.
 */
abstract class NamedCompilationUnitMemberImpl extends CompilationUnitMemberImpl
    implements NamedCompilationUnitMember {
  /**
   * The name of the member being declared.
   */
  SimpleIdentifierImpl _name;

  /**
   * Initialize a newly created compilation unit member with the given [name].
   * Either or both of the [comment] and [metadata] can be `null` if the member
   * does not have the corresponding attribute.
   */
  NamedCompilationUnitMemberImpl(
      CommentImpl comment, List<Annotation> metadata, SimpleIdentifierImpl name)
      : super(comment, metadata) {
    _name = _becomeParentOf(name);
  }

  @override
  SimpleIdentifier get name => _name;

  @override
  void set name(SimpleIdentifier identifier) {
    _name = _becomeParentOf(identifier as SimpleIdentifierImpl);
  }
}

/**
 * An expression that has a name associated with it. They are used in method
 * invocations when there are named parameters.
 *
 *    namedExpression ::=
 *        [Label] [Expression]
 */
class NamedExpressionImpl extends ExpressionImpl implements NamedExpression {
  /**
   * The name associated with the expression.
   */
  LabelImpl _name;

  /**
   * The expression with which the name is associated.
   */
  ExpressionImpl _expression;

  /**
   * Initialize a newly created named expression..
   */
  NamedExpressionImpl(LabelImpl name, ExpressionImpl expression) {
    _name = _becomeParentOf(name);
    _expression = _becomeParentOf(expression);
  }

  @override
  Token get beginToken => _name.beginToken;

  @override
  Iterable<SyntacticEntity> get childEntities =>
      new ChildEntities()..add(_name)..add(_expression);

  @override
  ParameterElement get element {
    Element element = _name.label.staticElement;
    if (element is ParameterElement) {
      return element;
    }
    return null;
  }

  @override
  Token get endToken => _expression.endToken;

  @override
  Expression get expression => _expression;

  @override
  void set expression(Expression expression) {
    _expression = _becomeParentOf(expression as ExpressionImpl);
  }

  @override
  Label get name => _name;

  @override
  void set name(Label identifier) {
    _name = _becomeParentOf(identifier as LabelImpl);
  }

  @override
  int get precedence => 0;

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitNamedExpression(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _name?.accept(visitor);
    _expression?.accept(visitor);
  }
}

/**
 * A node that represents a directive that impacts the namespace of a library.
 *
 *    directive ::=
 *        [ExportDirective]
 *      | [ImportDirective]
 */
abstract class NamespaceDirectiveImpl extends UriBasedDirectiveImpl
    implements NamespaceDirective {
  /**
   * The token representing the 'import' or 'export' keyword.
   */
  @override
  Token keyword;

  /**
   * The configurations used to control which library will actually be loaded at
   * run-time.
   */
  NodeList<Configuration> _configurations;

  /**
   * The combinators used to control which names are imported or exported.
   */
  NodeList<Combinator> _combinators;

  /**
   * The semicolon terminating the directive.
   */
  @override
  Token semicolon;

  @override
  String selectedUriContent;

  @override
  Source selectedSource;

  /**
   * Initialize a newly created namespace directive. Either or both of the
   * [comment] and [metadata] can be `null` if the directive does not have the
   * corresponding attribute. The list of [combinators] can be `null` if there
   * are no combinators.
   */
  NamespaceDirectiveImpl(
      CommentImpl comment,
      List<Annotation> metadata,
      this.keyword,
      StringLiteralImpl libraryUri,
      List<Configuration> configurations,
      List<Combinator> combinators,
      this.semicolon)
      : super(comment, metadata, libraryUri) {
    _configurations = new NodeListImpl<Configuration>(this, configurations);
    _combinators = new NodeListImpl<Combinator>(this, combinators);
  }

  @override
  NodeList<Combinator> get combinators => _combinators;

  @override
  NodeList<Configuration> get configurations => _configurations;

  @override
  Token get endToken => semicolon;

  @override
  Token get firstTokenAfterCommentAndMetadata => keyword;

  @deprecated
  @override
  Source get source => selectedSource;

  @deprecated
  @override
  void set source(Source source) {
    selectedSource = source;
  }

  @override
  LibraryElement get uriElement;
}

/**
 * The "native" clause in an class declaration.
 *
 *    nativeClause ::=
 *        'native' [StringLiteral]
 */
class NativeClauseImpl extends AstNodeImpl implements NativeClause {
  /**
   * The token representing the 'native' keyword.
   */
  @override
  Token nativeKeyword;

  /**
   * The name of the native object that implements the class.
   */
  StringLiteralImpl _name;

  /**
   * Initialize a newly created native clause.
   */
  NativeClauseImpl(this.nativeKeyword, StringLiteralImpl name) {
    _name = _becomeParentOf(name);
  }

  @override
  Token get beginToken => nativeKeyword;

  @override
  Iterable<SyntacticEntity> get childEntities =>
      new ChildEntities()..add(nativeKeyword)..add(_name);

  @override
  Token get endToken => _name.endToken;

  @override
  StringLiteral get name => _name;

  @override
  void set name(StringLiteral name) {
    _name = _becomeParentOf(name as StringLiteralImpl);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitNativeClause(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _name?.accept(visitor);
  }
}

/**
 * A function body that consists of a native keyword followed by a string
 * literal.
 *
 *    nativeFunctionBody ::=
 *        'native' [SimpleStringLiteral] ';'
 */
class NativeFunctionBodyImpl extends FunctionBodyImpl
    implements NativeFunctionBody {
  /**
   * The token representing 'native' that marks the start of the function body.
   */
  @override
  Token nativeKeyword;

  /**
   * The string literal, after the 'native' token.
   */
  StringLiteralImpl _stringLiteral;

  /**
   * The token representing the semicolon that marks the end of the function
   * body.
   */
  @override
  Token semicolon;

  /**
   * Initialize a newly created function body consisting of the 'native' token,
   * a string literal, and a semicolon.
   */
  NativeFunctionBodyImpl(
      this.nativeKeyword, StringLiteralImpl stringLiteral, this.semicolon) {
    _stringLiteral = _becomeParentOf(stringLiteral);
  }

  @override
  Token get beginToken => nativeKeyword;

  @override
  Iterable<SyntacticEntity> get childEntities => new ChildEntities()
    ..add(nativeKeyword)
    ..add(_stringLiteral)
    ..add(semicolon);

  @override
  Token get endToken => semicolon;

  @override
  StringLiteral get stringLiteral => _stringLiteral;

  @override
  void set stringLiteral(StringLiteral stringLiteral) {
    _stringLiteral = _becomeParentOf(stringLiteral as StringLiteralImpl);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitNativeFunctionBody(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _stringLiteral?.accept(visitor);
  }
}

/**
 * A list of AST nodes that have a common parent.
 */
class NodeListImpl<E extends AstNode> with ListMixin<E> implements NodeList<E> {
  /**
   * The node that is the parent of each of the elements in the list.
   */
  AstNodeImpl _owner;

  /**
   * The elements contained in the list.
   */
  List<E> _elements = <E>[];

  /**
   * Initialize a newly created list of nodes such that all of the nodes that
   * are added to the list will have their parent set to the given [owner]. The
   * list will initially be populated with the given [elements].
   */
  NodeListImpl(this._owner, [List<E> elements]) {
    addAll(elements);
  }

  @override
  Token get beginToken {
    if (_elements.length == 0) {
      return null;
    }
    return _elements[0].beginToken;
  }

  @override
  Token get endToken {
    int length = _elements.length;
    if (length == 0) {
      return null;
    }
    return _elements[length - 1].endToken;
  }

  int get length => _elements.length;

  @deprecated // Never intended for public use.
  @override
  void set length(int newLength) {
    throw new UnsupportedError("Cannot resize NodeList.");
  }

  @override
  AstNode get owner => _owner;

  @override
  void set owner(AstNode value) {
    _owner = value as AstNodeImpl;
  }

  E operator [](int index) {
    if (index < 0 || index >= _elements.length) {
      throw new RangeError("Index: $index, Size: ${_elements.length}");
    }
    return _elements[index];
  }

  void operator []=(int index, E node) {
    if (index < 0 || index >= _elements.length) {
      throw new RangeError("Index: $index, Size: ${_elements.length}");
    }
    _owner._becomeParentOf(node as AstNodeImpl);
    _elements[index] = node;
  }

  @override
  accept(AstVisitor visitor) {
    int length = _elements.length;
    for (var i = 0; i < length; i++) {
      _elements[i].accept(visitor);
    }
  }

  @override
  void add(E node) {
    insert(length, node);
  }

  @override
  bool addAll(Iterable<E> nodes) {
    if (nodes != null && !nodes.isEmpty) {
      if (nodes is List<E>) {
        int length = nodes.length;
        for (int i = 0; i < length; i++) {
          E node = nodes[i];
          _elements.add(node);
          _owner._becomeParentOf(node as AstNodeImpl);
        }
      } else {
        for (E node in nodes) {
          _elements.add(node);
          _owner._becomeParentOf(node as AstNodeImpl);
        }
      }
      return true;
    }
    return false;
  }

  @override
  void clear() {
    _elements = <E>[];
  }

  @override
  void insert(int index, E node) {
    int length = _elements.length;
    if (index < 0 || index > length) {
      throw new RangeError("Index: $index, Size: ${_elements.length}");
    }
    _owner._becomeParentOf(node as AstNodeImpl);
    if (length == 0) {
      _elements.add(node);
    } else {
      _elements.insert(index, node);
    }
  }

  @override
  E removeAt(int index) {
    if (index < 0 || index >= _elements.length) {
      throw new RangeError("Index: $index, Size: ${_elements.length}");
    }
    E removedNode = _elements[index];
    _elements.removeAt(index);
    return removedNode;
  }

  /// This is non-API and may be changed or removed at any point.
  ///
  /// Changes the length of this list
  /// If [newLength] is greater than the current length,
  /// entries are initialized to `null`.
  ///
  /// This list should NOT contain any `null` elements,
  /// so be sure to immediately follow a call to this method with calls
  /// to replace all the `null` elements with non-`null` elements.
  void setLength(int newLength) {
    _elements.length = newLength;
  }
}

/**
 * A formal parameter that is required (is not optional).
 *
 *    normalFormalParameter ::=
 *        [FunctionTypedFormalParameter]
 *      | [FieldFormalParameter]
 *      | [SimpleFormalParameter]
 */
abstract class NormalFormalParameterImpl extends FormalParameterImpl
    implements NormalFormalParameter {
  /**
   * The documentation comment associated with this parameter, or `null` if this
   * parameter does not have a documentation comment associated with it.
   */
  CommentImpl _comment;

  /**
   * The annotations associated with this parameter.
   */
  NodeList<Annotation> _metadata;

  /**
   * The 'covariant' keyword, or `null` if the keyword was not used.
   */
  Token covariantKeyword;

  /**
   * The name of the parameter being declared.
   */
  SimpleIdentifierImpl _identifier;

  /**
   * Initialize a newly created formal parameter. Either or both of the
   * [comment] and [metadata] can be `null` if the parameter does not have the
   * corresponding attribute.
   */
  NormalFormalParameterImpl(CommentImpl comment, List<Annotation> metadata,
      this.covariantKeyword, SimpleIdentifierImpl identifier) {
    _comment = _becomeParentOf(comment);
    _metadata = new NodeListImpl<Annotation>(this, metadata);
    _identifier = _becomeParentOf(identifier);
  }

  @override
  Comment get documentationComment => _comment;

  @override
  void set documentationComment(Comment comment) {
    _comment = _becomeParentOf(comment as CommentImpl);
  }

  @override
  SimpleIdentifier get identifier => _identifier;

  @override
  void set identifier(SimpleIdentifier identifier) {
    _identifier = _becomeParentOf(identifier as SimpleIdentifierImpl);
  }

  @deprecated
  @override
  ParameterKind get kind {
    AstNode parent = this.parent;
    if (parent is DefaultFormalParameter) {
      return parent.kind;
    }
    return ParameterKind.REQUIRED;
  }

  @override
  NodeList<Annotation> get metadata => _metadata;

  @override
  void set metadata(List<Annotation> metadata) {
    _metadata.clear();
    _metadata.addAll(metadata);
  }

  @override
  List<AstNode> get sortedCommentAndAnnotations {
    return <AstNode>[]
      ..add(_comment)
      ..addAll(_metadata)
      ..sort(AstNode.LEXICAL_ORDER);
  }

  ChildEntities get _childEntities {
    ChildEntities result = new ChildEntities();
    if (_commentIsBeforeAnnotations()) {
      result
        ..add(_comment)
        ..addAll(_metadata);
    } else {
      result.addAll(sortedCommentAndAnnotations);
    }
    if (covariantKeyword != null) {
      result.add(covariantKeyword);
    }
    return result;
  }

  @override
  void visitChildren(AstVisitor visitor) {
    //
    // Note that subclasses are responsible for visiting the identifier because
    // they often need to visit other nodes before visiting the identifier.
    //
    if (_commentIsBeforeAnnotations()) {
      _comment?.accept(visitor);
      _metadata.accept(visitor);
    } else {
      List<AstNode> children = sortedCommentAndAnnotations;
      int length = children.length;
      for (int i = 0; i < length; i++) {
        children[i].accept(visitor);
      }
    }
  }

  /**
   * Return `true` if the comment is lexically before any annotations.
   */
  bool _commentIsBeforeAnnotations() {
    if (_comment == null || _metadata.isEmpty) {
      return true;
    }
    Annotation firstAnnotation = _metadata[0];
    return _comment.offset < firstAnnotation.offset;
  }
}

/**
 * A null literal expression.
 *
 *    nullLiteral ::=
 *        'null'
 */
class NullLiteralImpl extends LiteralImpl implements NullLiteral {
  /**
   * The token representing the literal.
   */
  Token literal;

  /**
   * Initialize a newly created null literal.
   */
  NullLiteralImpl(this.literal);

  @override
  Token get beginToken => literal;

  @override
  Iterable<SyntacticEntity> get childEntities =>
      new ChildEntities()..add(literal);

  @override
  Token get endToken => literal;

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitNullLiteral(this);

  @override
  void visitChildren(AstVisitor visitor) {
    // There are no children to visit.
  }
}

/**
 * The "on" clause in a mixin declaration.
 *
 *    onClause ::=
 *        'on' [TypeName] (',' [TypeName])*
 */
class OnClauseImpl extends AstNodeImpl implements OnClause {
  @override
  Token onKeyword;

  /**
   * The classes are super-class constraints for the mixin.
   */
  NodeList<TypeName> _superclassConstraints;

  /**
   * Initialize a newly created on clause.
   */
  OnClauseImpl(this.onKeyword, List<TypeName> superclassConstraints) {
    _superclassConstraints =
        new NodeListImpl<TypeName>(this, superclassConstraints);
  }

  @override
  Token get beginToken => onKeyword;

  @override
  // TODO(paulberry): add commas.
  Iterable<SyntacticEntity> get childEntities => new ChildEntities()
    ..add(onKeyword)
    ..addAll(superclassConstraints);

  @override
  Token get endToken => _superclassConstraints.endToken;

  @override
  NodeList<TypeName> get superclassConstraints => _superclassConstraints;

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitOnClause(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _superclassConstraints.accept(visitor);
  }
}

/**
 * A parenthesized expression.
 *
 *    parenthesizedExpression ::=
 *        '(' [Expression] ')'
 */
class ParenthesizedExpressionImpl extends ExpressionImpl
    implements ParenthesizedExpression {
  /**
   * The left parenthesis.
   */
  Token leftParenthesis;

  /**
   * The expression within the parentheses.
   */
  ExpressionImpl _expression;

  /**
   * The right parenthesis.
   */
  Token rightParenthesis;

  /**
   * Initialize a newly created parenthesized expression.
   */
  ParenthesizedExpressionImpl(
      this.leftParenthesis, ExpressionImpl expression, this.rightParenthesis) {
    _expression = _becomeParentOf(expression);
  }

  @override
  Token get beginToken => leftParenthesis;

  @override
  Iterable<SyntacticEntity> get childEntities => new ChildEntities()
    ..add(leftParenthesis)
    ..add(_expression)
    ..add(rightParenthesis);

  @override
  Token get endToken => rightParenthesis;

  @override
  Expression get expression => _expression;

  @override
  void set expression(Expression expression) {
    _expression = _becomeParentOf(expression as ExpressionImpl);
  }

  @override
  int get precedence => 15;

  @override
  Expression get unParenthesized {
    // This is somewhat inefficient, but it avoids a stack overflow in the
    // degenerate case.
    Expression expression = _expression;
    while (expression is ParenthesizedExpressionImpl) {
      expression = (expression as ParenthesizedExpressionImpl)._expression;
    }
    return expression;
  }

  @override
  E accept<E>(AstVisitor<E> visitor) =>
      visitor.visitParenthesizedExpression(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _expression?.accept(visitor);
  }
}

/**
 * A part directive.
 *
 *    partDirective ::=
 *        [Annotation] 'part' [StringLiteral] ';'
 */
class PartDirectiveImpl extends UriBasedDirectiveImpl implements PartDirective {
  /**
   * The token representing the 'part' keyword.
   */
  @override
  Token partKeyword;

  /**
   * The semicolon terminating the directive.
   */
  @override
  Token semicolon;

  /**
   * Initialize a newly created part directive. Either or both of the [comment]
   * and [metadata] can be `null` if the directive does not have the
   * corresponding attribute.
   */
  PartDirectiveImpl(CommentImpl comment, List<Annotation> metadata,
      this.partKeyword, StringLiteralImpl partUri, this.semicolon)
      : super(comment, metadata, partUri);

  @override
  Iterable<SyntacticEntity> get childEntities =>
      super._childEntities..add(partKeyword)..add(_uri)..add(semicolon);

  @override
  Token get endToken => semicolon;

  @override
  Token get firstTokenAfterCommentAndMetadata => partKeyword;

  @override
  Token get keyword => partKeyword;

  @override
  CompilationUnitElement get uriElement => element as CompilationUnitElement;

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitPartDirective(this);
}

/**
 * A part-of directive.
 *
 *    partOfDirective ::=
 *        [Annotation] 'part' 'of' [Identifier] ';'
 */
class PartOfDirectiveImpl extends DirectiveImpl implements PartOfDirective {
  /**
   * The token representing the 'part' keyword.
   */
  @override
  Token partKeyword;

  /**
   * The token representing the 'of' keyword.
   */
  @override
  Token ofKeyword;

  /**
   * The URI of the library that the containing compilation unit is part of.
   */
  StringLiteralImpl _uri;

  /**
   * The name of the library that the containing compilation unit is part of, or
   * `null` if no name was given (typically because a library URI was provided).
   */
  LibraryIdentifierImpl _libraryName;

  /**
   * The semicolon terminating the directive.
   */
  @override
  Token semicolon;

  /**
   * Initialize a newly created part-of directive. Either or both of the
   * [comment] and [metadata] can be `null` if the directive does not have the
   * corresponding attribute.
   */
  PartOfDirectiveImpl(
      CommentImpl comment,
      List<Annotation> metadata,
      this.partKeyword,
      this.ofKeyword,
      StringLiteralImpl uri,
      LibraryIdentifierImpl libraryName,
      this.semicolon)
      : super(comment, metadata) {
    _uri = _becomeParentOf(uri);
    _libraryName = _becomeParentOf(libraryName);
  }

  @override
  Iterable<SyntacticEntity> get childEntities => super._childEntities
    ..add(partKeyword)
    ..add(ofKeyword)
    ..add(_uri)
    ..add(_libraryName)
    ..add(semicolon);

  @override
  Token get endToken => semicolon;

  @override
  Token get firstTokenAfterCommentAndMetadata => partKeyword;

  @override
  Token get keyword => partKeyword;

  @override
  LibraryIdentifier get libraryName => _libraryName;

  @override
  void set libraryName(LibraryIdentifier libraryName) {
    _libraryName = _becomeParentOf(libraryName as LibraryIdentifierImpl);
  }

  @override
  StringLiteral get uri => _uri;

  @override
  void set uri(StringLiteral uri) {
    _uri = _becomeParentOf(uri as StringLiteralImpl);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitPartOfDirective(this);

  @override
  void visitChildren(AstVisitor visitor) {
    super.visitChildren(visitor);
    _libraryName?.accept(visitor);
    _uri?.accept(visitor);
  }
}

/**
 * A postfix unary expression.
 *
 *    postfixExpression ::=
 *        [Expression] [Token]
 */
class PostfixExpressionImpl extends ExpressionImpl
    implements PostfixExpression {
  /**
   * The expression computing the operand for the operator.
   */
  ExpressionImpl _operand;

  /**
   * The postfix operator being applied to the operand.
   */
  @override
  Token operator;

  /**
   * The element associated with the operator based on the static type of the
   * operand, or `null` if the AST structure has not been resolved, if the
   * operator is not user definable, or if the operator could not be resolved.
   */
  @override
  MethodElement staticElement;

  /**
   * Initialize a newly created postfix expression.
   */
  PostfixExpressionImpl(ExpressionImpl operand, this.operator) {
    _operand = _becomeParentOf(operand);
  }

  @override
  Token get beginToken => _operand.beginToken;

  @override
  @deprecated
  MethodElement get bestElement => staticElement;

  @override
  Iterable<SyntacticEntity> get childEntities =>
      new ChildEntities()..add(_operand)..add(operator);

  @override
  Token get endToken => operator;

  @override
  Expression get operand => _operand;

  @override
  void set operand(Expression expression) {
    _operand = _becomeParentOf(expression as ExpressionImpl);
  }

  @override
  int get precedence => 15;

  @deprecated
  @override
  MethodElement get propagatedElement => null;

  @deprecated
  @override
  set propagatedElement(MethodElement element) {}

  /**
   * If the AST structure has been resolved, and the function being invoked is
   * known based on static type information, then return the parameter element
   * representing the parameter to which the value of the operand will be bound.
   * Otherwise, return `null`.
   */
  ParameterElement get _staticParameterElementForOperand {
    if (staticElement == null) {
      return null;
    }
    List<ParameterElement> parameters = staticElement.parameters;
    if (parameters.length < 1) {
      return null;
    }
    return parameters[0];
  }

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitPostfixExpression(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _operand?.accept(visitor);
  }
}

/**
 * An identifier that is prefixed or an access to an object property where the
 * target of the property access is a simple identifier.
 *
 *    prefixedIdentifier ::=
 *        [SimpleIdentifier] '.' [SimpleIdentifier]
 */
class PrefixedIdentifierImpl extends IdentifierImpl
    implements PrefixedIdentifier {
  /**
   * The prefix associated with the library in which the identifier is defined.
   */
  SimpleIdentifierImpl _prefix;

  /**
   * The period used to separate the prefix from the identifier.
   */
  Token period;

  /**
   * The identifier being prefixed.
   */
  SimpleIdentifierImpl _identifier;

  /**
   * Initialize a newly created prefixed identifier.
   */
  PrefixedIdentifierImpl(SimpleIdentifierImpl prefix, this.period,
      SimpleIdentifierImpl identifier) {
    _prefix = _becomeParentOf(prefix);
    _identifier = _becomeParentOf(identifier);
  }

  /**
   * Initialize a newly created prefixed identifier that does not take ownership
   * of the components. The resulting node is only for temporary use, such as by
   * resolution.
   */
  PrefixedIdentifierImpl.temp(this._prefix, this._identifier) : period = null;

  @override
  Token get beginToken => _prefix.beginToken;

  @override
  @deprecated
  Element get bestElement {
    if (_identifier == null) {
      return null;
    }
    return _identifier.staticElement;
  }

  @override
  Iterable<SyntacticEntity> get childEntities =>
      new ChildEntities()..add(_prefix)..add(period)..add(_identifier);

  @override
  Token get endToken => _identifier.endToken;

  @override
  SimpleIdentifier get identifier => _identifier;

  @override
  void set identifier(SimpleIdentifier identifier) {
    _identifier = _becomeParentOf(identifier as SimpleIdentifierImpl);
  }

  @override
  bool get isDeferred {
    Element element = _prefix.staticElement;
    if (element is PrefixElement) {
      List<ImportElement> imports =
          element.enclosingElement.getImportsWithPrefix(element);
      if (imports.length != 1) {
        return false;
      }
      return imports[0].isDeferred;
    }
    return false;
  }

  @override
  String get name => "${_prefix.name}.${_identifier.name}";

  @override
  int get precedence => 15;

  @override
  SimpleIdentifier get prefix => _prefix;

  @override
  void set prefix(SimpleIdentifier identifier) {
    _prefix = _becomeParentOf(identifier as SimpleIdentifierImpl);
  }

  @deprecated
  @override
  Element get propagatedElement => null;

  @override
  Element get staticElement {
    if (_identifier == null) {
      return null;
    }
    return _identifier.staticElement;
  }

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitPrefixedIdentifier(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _prefix?.accept(visitor);
    _identifier?.accept(visitor);
  }
}

/**
 * A prefix unary expression.
 *
 *    prefixExpression ::=
 *        [Token] [Expression]
 */
class PrefixExpressionImpl extends ExpressionImpl implements PrefixExpression {
  /**
   * The prefix operator being applied to the operand.
   */
  Token operator;

  /**
   * The expression computing the operand for the operator.
   */
  ExpressionImpl _operand;

  /**
   * The element associated with the operator based on the static type of the
   * operand, or `null` if the AST structure has not been resolved, if the
   * operator is not user definable, or if the operator could not be resolved.
   */
  MethodElement staticElement;

  /**
   * Initialize a newly created prefix expression.
   */
  PrefixExpressionImpl(this.operator, ExpressionImpl operand) {
    _operand = _becomeParentOf(operand);
  }

  @override
  Token get beginToken => operator;

  @override
  @deprecated
  MethodElement get bestElement => staticElement;

  @override
  Iterable<SyntacticEntity> get childEntities =>
      new ChildEntities()..add(operator)..add(_operand);

  @override
  Token get endToken => _operand.endToken;

  @override
  Expression get operand => _operand;

  @override
  void set operand(Expression expression) {
    _operand = _becomeParentOf(expression as ExpressionImpl);
  }

  @override
  int get precedence => 14;

  @deprecated
  @override
  MethodElement get propagatedElement => null;

  @deprecated
  @override
  set propagatedElement(MethodElement element) {}

  /**
   * If the AST structure has been resolved, and the function being invoked is
   * known based on static type information, then return the parameter element
   * representing the parameter to which the value of the operand will be bound.
   * Otherwise, return `null`.
   */
  ParameterElement get _staticParameterElementForOperand {
    if (staticElement == null) {
      return null;
    }
    List<ParameterElement> parameters = staticElement.parameters;
    if (parameters.length < 1) {
      return null;
    }
    return parameters[0];
  }

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitPrefixExpression(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _operand?.accept(visitor);
  }
}

/**
 * The access of a property of an object.
 *
 * Note, however, that accesses to properties of objects can also be represented
 * as [PrefixedIdentifier] nodes in cases where the target is also a simple
 * identifier.
 *
 *    propertyAccess ::=
 *        [Expression] '.' [SimpleIdentifier]
 */
class PropertyAccessImpl extends ExpressionImpl implements PropertyAccess {
  /**
   * The expression computing the object defining the property being accessed.
   */
  ExpressionImpl _target;

  /**
   * The property access operator.
   */
  Token operator;

  /**
   * The name of the property being accessed.
   */
  SimpleIdentifierImpl _propertyName;

  /**
   * Initialize a newly created property access expression.
   */
  PropertyAccessImpl(
      ExpressionImpl target, this.operator, SimpleIdentifierImpl propertyName) {
    _target = _becomeParentOf(target);
    _propertyName = _becomeParentOf(propertyName);
  }

  @override
  Token get beginToken {
    if (_target != null) {
      return _target.beginToken;
    }
    return operator;
  }

  @override
  Iterable<SyntacticEntity> get childEntities =>
      new ChildEntities()..add(_target)..add(operator)..add(_propertyName);

  @override
  Token get endToken => _propertyName.endToken;

  @override
  bool get isAssignable => true;

  @override
  bool get isCascaded =>
      operator != null && operator.type == TokenType.PERIOD_PERIOD;

  @override
  int get precedence => 15;

  @override
  SimpleIdentifier get propertyName => _propertyName;

  @override
  void set propertyName(SimpleIdentifier identifier) {
    _propertyName = _becomeParentOf(identifier as SimpleIdentifierImpl);
  }

  @override
  Expression get realTarget {
    if (isCascaded) {
      AstNode ancestor = parent;
      while (ancestor is! CascadeExpression) {
        if (ancestor == null) {
          return _target;
        }
        ancestor = ancestor.parent;
      }
      return (ancestor as CascadeExpression).target;
    }
    return _target;
  }

  @override
  Expression get target => _target;

  @override
  void set target(Expression expression) {
    _target = _becomeParentOf(expression as ExpressionImpl);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitPropertyAccess(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _target?.accept(visitor);
    _propertyName?.accept(visitor);
  }
}

/**
 * The invocation of a constructor in the same class from within a constructor's
 * initialization list.
 *
 *    redirectingConstructorInvocation ::=
 *        'this' ('.' identifier)? arguments
 */
class RedirectingConstructorInvocationImpl extends ConstructorInitializerImpl
    implements RedirectingConstructorInvocation {
  /**
   * The token for the 'this' keyword.
   */
  Token thisKeyword;

  /**
   * The token for the period before the name of the constructor that is being
   * invoked, or `null` if the unnamed constructor is being invoked.
   */
  Token period;

  /**
   * The name of the constructor that is being invoked, or `null` if the unnamed
   * constructor is being invoked.
   */
  SimpleIdentifierImpl _constructorName;

  /**
   * The list of arguments to the constructor.
   */
  ArgumentListImpl _argumentList;

  /**
   * The element associated with the constructor based on static type
   * information, or `null` if the AST structure has not been resolved or if the
   * constructor could not be resolved.
   */
  ConstructorElement staticElement;

  /**
   * Initialize a newly created redirecting invocation to invoke the constructor
   * with the given name with the given arguments. The [constructorName] can be
   * `null` if the constructor being invoked is the unnamed constructor.
   */
  RedirectingConstructorInvocationImpl(this.thisKeyword, this.period,
      SimpleIdentifierImpl constructorName, ArgumentListImpl argumentList) {
    _constructorName = _becomeParentOf(constructorName);
    _argumentList = _becomeParentOf(argumentList);
  }

  @override
  ArgumentList get argumentList => _argumentList;

  @override
  void set argumentList(ArgumentList argumentList) {
    _argumentList = _becomeParentOf(argumentList as ArgumentListImpl);
  }

  @override
  Token get beginToken => thisKeyword;

  @override
  Iterable<SyntacticEntity> get childEntities => new ChildEntities()
    ..add(thisKeyword)
    ..add(period)
    ..add(_constructorName)
    ..add(_argumentList);

  @override
  SimpleIdentifier get constructorName => _constructorName;

  @override
  void set constructorName(SimpleIdentifier identifier) {
    _constructorName = _becomeParentOf(identifier as SimpleIdentifierImpl);
  }

  @override
  Token get endToken => _argumentList.endToken;

  @override
  E accept<E>(AstVisitor<E> visitor) =>
      visitor.visitRedirectingConstructorInvocation(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _constructorName?.accept(visitor);
    _argumentList?.accept(visitor);
  }
}

/**
 * A rethrow expression.
 *
 *    rethrowExpression ::=
 *        'rethrow'
 */
class RethrowExpressionImpl extends ExpressionImpl
    implements RethrowExpression {
  /**
   * The token representing the 'rethrow' keyword.
   */
  Token rethrowKeyword;

  /**
   * Initialize a newly created rethrow expression.
   */
  RethrowExpressionImpl(this.rethrowKeyword);

  @override
  Token get beginToken => rethrowKeyword;

  @override
  Iterable<SyntacticEntity> get childEntities =>
      new ChildEntities()..add(rethrowKeyword);

  @override
  Token get endToken => rethrowKeyword;

  @override
  int get precedence => 0;

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitRethrowExpression(this);

  @override
  void visitChildren(AstVisitor visitor) {
    // There are no children to visit.
  }
}

/**
 * A return statement.
 *
 *    returnStatement ::=
 *        'return' [Expression]? ';'
 */
class ReturnStatementImpl extends StatementImpl implements ReturnStatement {
  /**
   * The token representing the 'return' keyword.
   */
  Token returnKeyword;

  /**
   * The expression computing the value to be returned, or `null` if no explicit
   * value was provided.
   */
  ExpressionImpl _expression;

  /**
   * The semicolon terminating the statement.
   */
  Token semicolon;

  /**
   * Initialize a newly created return statement. The [expression] can be `null`
   * if no explicit value was provided.
   */
  ReturnStatementImpl(
      this.returnKeyword, ExpressionImpl expression, this.semicolon) {
    _expression = _becomeParentOf(expression);
  }

  @override
  Token get beginToken => returnKeyword;

  @override
  Iterable<SyntacticEntity> get childEntities =>
      new ChildEntities()..add(returnKeyword)..add(_expression)..add(semicolon);

  @override
  Token get endToken => semicolon;

  @override
  Expression get expression => _expression;

  @override
  void set expression(Expression expression) {
    _expression = _becomeParentOf(expression as ExpressionImpl);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitReturnStatement(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _expression?.accept(visitor);
  }
}

/**
 * A script tag that can optionally occur at the beginning of a compilation unit.
 *
 *    scriptTag ::=
 *        '#!' (~NEWLINE)* NEWLINE
 */
class ScriptTagImpl extends AstNodeImpl implements ScriptTag {
  /**
   * The token representing this script tag.
   */
  Token scriptTag;

  /**
   * Initialize a newly created script tag.
   */
  ScriptTagImpl(this.scriptTag);

  @override
  Token get beginToken => scriptTag;

  @override
  Iterable<SyntacticEntity> get childEntities =>
      new ChildEntities()..add(scriptTag);

  @override
  Token get endToken => scriptTag;

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitScriptTag(this);

  @override
  void visitChildren(AstVisitor visitor) {
    // There are no children to visit.
  }
}

/**
 * A literal set.
 *
 *    setLiteral ::=
 *        'const'? ('<' [TypeAnnotation] '>')?
 *        '{' [CollectionElement] (',' [Expression])* ','? '}'
 *      | 'const'? ('<' [TypeAnnotation] '>')? '{' '}'
 *
 * This is the class that is used to represent a set literal when either the
 * 'control-flow-collections' or 'spread-collections' experiments are enabled.
 * If neither of those experiments are enabled, then [SetLiteral] will be used.
 */
class SetLiteral2Impl extends TypedLiteralImpl implements SetLiteral2 {
  @override
  Token leftBracket;

  /**
   * The elements in the set.
   */
  NodeList<CollectionElement> _elements;

  @override
  Token rightBracket;

  /**
   * Initialize a newly created set literal. The [constKeyword] can be `null` if
   * the literal is not a constant. The [typeArguments] can be `null` if no type
   * arguments were declared. The [elements] can be `null` if the set is empty.
   */
  SetLiteral2Impl(Token constKeyword, TypeArgumentListImpl typeArguments,
      this.leftBracket, List<CollectionElement> elements, this.rightBracket)
      : super(constKeyword, typeArguments) {
    _elements = new NodeListImpl<CollectionElement>(this, elements);
  }

  @override
  Token get beginToken {
    if (constKeyword != null) {
      return constKeyword;
    }
    TypeArgumentList typeArguments = this.typeArguments;
    if (typeArguments != null) {
      return typeArguments.beginToken;
    }
    return leftBracket;
  }

  @override
  // TODO(paulberry): add commas.
  Iterable<SyntacticEntity> get childEntities => super._childEntities
    ..add(leftBracket)
    ..addAll(elements)
    ..add(rightBracket);

  @override
  NodeList<CollectionElement> get elements => _elements;

  @override
  Token get endToken => rightBracket;

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitSetLiteral2(this);

  @override
  void visitChildren(AstVisitor visitor) {
    super.visitChildren(visitor);
    _elements.accept(visitor);
  }
}

/**
 * A literal set.
 *
 *    setLiteral ::=
 *        'const'? ('<' [TypeAnnotation] '>')?
 *        '{' [Expression] (',' [Expression])* ','? '}'
 *      | 'const'? ('<' [TypeAnnotation] '>')? '{' '}'
 *
 * This is the class that is used to represent a set literal when neither the
 * 'control-flow-collections' nor 'spread-collections' experiments are enabled.
 * If either of those experiments are enabled, then [SetLiteral2] will be used.
 */
class SetLiteralImpl extends TypedLiteralImpl implements SetLiteral {
  /**
   * The left curly bracket.
   */
  @override
  Token leftBracket;

  /**
   * The elements in the set.
   */
  NodeList<Expression> _elements;

  /**
   * The right curly bracket.
   */
  @override
  Token rightBracket;

  /**
   * Initialize a newly created set literal. The [constKeyword] can be `null` if
   * the literal is not a constant. The [typeArguments] can be `null` if no type
   * arguments were declared. The [elements] can be `null` if the set is empty.
   */
  SetLiteralImpl(Token constKeyword, TypeArgumentListImpl typeArguments,
      this.leftBracket, List<Expression> elements, this.rightBracket)
      : super(constKeyword, typeArguments) {
    _elements = new NodeListImpl<Expression>(this, elements);
  }

  @override
  Token get beginToken {
    if (constKeyword != null) {
      return constKeyword;
    }
    TypeArgumentList typeArguments = this.typeArguments;
    if (typeArguments != null) {
      return typeArguments.beginToken;
    }
    return leftBracket;
  }

  @override
  // TODO(paulberry): add commas.
  Iterable<SyntacticEntity> get childEntities => super._childEntities
    ..add(leftBracket)
    ..addAll(elements)
    ..add(rightBracket);

  @override
  NodeList<Expression> get elements => _elements;

  @override
  Token get endToken => rightBracket;

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitSetLiteral(this);

  @override
  void visitChildren(AstVisitor visitor) {
    super.visitChildren(visitor);
    _elements.accept(visitor);
  }
}

/**
 * A combinator that restricts the names being imported to those in a given list.
 *
 *    showCombinator ::=
 *        'show' [SimpleIdentifier] (',' [SimpleIdentifier])*
 */
class ShowCombinatorImpl extends CombinatorImpl implements ShowCombinator {
  /**
   * The list of names from the library that are made visible by this combinator.
   */
  NodeList<SimpleIdentifier> _shownNames;

  /**
   * Initialize a newly created import show combinator.
   */
  ShowCombinatorImpl(Token keyword, List<SimpleIdentifier> shownNames)
      : super(keyword) {
    _shownNames = new NodeListImpl<SimpleIdentifier>(this, shownNames);
  }

  @override
  // TODO(paulberry): add commas.
  Iterable<SyntacticEntity> get childEntities => new ChildEntities()
    ..add(keyword)
    ..addAll(_shownNames);

  @override
  Token get endToken => _shownNames.endToken;

  @override
  NodeList<SimpleIdentifier> get shownNames => _shownNames;

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitShowCombinator(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _shownNames.accept(visitor);
  }
}

/**
 * A simple formal parameter.
 *
 *    simpleFormalParameter ::=
 *        ('final' [TypeName] | 'var' | [TypeName])? [SimpleIdentifier]
 */
class SimpleFormalParameterImpl extends NormalFormalParameterImpl
    implements SimpleFormalParameter {
  /**
   * The token representing either the 'final', 'const' or 'var' keyword, or
   * `null` if no keyword was used.
   */
  Token keyword;

  /**
   * The name of the declared type of the parameter, or `null` if the parameter
   * does not have a declared type.
   */
  TypeAnnotationImpl _type;

  @override
  // TODO(brianwilkerson) This overrides a concrete implementation in which the
  // element is assumed to be stored in the `identifier`, but there is no
  // corresponding inherited setter. This seems inconsistent and error prone.
  ParameterElement declaredElement;

  /**
   * Initialize a newly created formal parameter. Either or both of the
   * [comment] and [metadata] can be `null` if the parameter does not have the
   * corresponding attribute. The [keyword] can be `null` if a type was
   * specified. The [type] must be `null` if the keyword is 'var'.
   */
  SimpleFormalParameterImpl(
      CommentImpl comment,
      List<Annotation> metadata,
      Token covariantKeyword,
      this.keyword,
      TypeAnnotationImpl type,
      SimpleIdentifierImpl identifier)
      : super(comment, metadata, covariantKeyword, identifier) {
    _type = _becomeParentOf(type);
  }

  @override
  Token get beginToken {
    NodeList<Annotation> metadata = this.metadata;
    if (!metadata.isEmpty) {
      return metadata.beginToken;
    } else if (covariantKeyword != null) {
      return covariantKeyword;
    } else if (keyword != null) {
      return keyword;
    } else if (_type != null) {
      return _type.beginToken;
    }
    return identifier?.beginToken;
  }

  @override
  Iterable<SyntacticEntity> get childEntities =>
      super._childEntities..add(keyword)..add(_type)..add(identifier);

  @override
  Token get endToken => identifier?.endToken ?? type?.endToken;

  @override
  bool get isConst => keyword?.keyword == Keyword.CONST;

  @override
  bool get isFinal => keyword?.keyword == Keyword.FINAL;

  @override
  TypeAnnotation get type => _type;

  @override
  void set type(TypeAnnotation type) {
    _type = _becomeParentOf(type as TypeAnnotationImpl);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) =>
      visitor.visitSimpleFormalParameter(this);

  @override
  void visitChildren(AstVisitor visitor) {
    super.visitChildren(visitor);
    _type?.accept(visitor);
    identifier?.accept(visitor);
  }
}

/**
 * A simple identifier.
 *
 *    simpleIdentifier ::=
 *        initialCharacter internalCharacter*
 *
 *    initialCharacter ::= '_' | '$' | letter
 *
 *    internalCharacter ::= '_' | '$' | letter | digit
 */
class SimpleIdentifierImpl extends IdentifierImpl implements SimpleIdentifier {
  /**
   * The token representing the identifier.
   */
  Token token;

  /**
   * The element associated with this identifier based on static type
   * information, or `null` if the AST structure has not been resolved or if
   * this identifier could not be resolved.
   */
  Element _staticElement;

  /**
   * If this expression is both in a getter and setter context, the
   * [AuxiliaryElements] will be set to hold onto the static element from the
   * getter context.
   */
  AuxiliaryElements auxiliaryElements = null;

  /**
   * Initialize a newly created identifier.
   */
  SimpleIdentifierImpl(this.token);

  @override
  Token get beginToken => token;

  @override
  @deprecated
  Element get bestElement => _staticElement;

  @override
  Iterable<SyntacticEntity> get childEntities =>
      new ChildEntities()..add(token);

  @override
  Token get endToken => token;

  @override
  bool get isQualified {
    AstNode parent = this.parent;
    if (parent is PrefixedIdentifier) {
      return identical(parent.identifier, this);
    } else if (parent is PropertyAccess) {
      return identical(parent.propertyName, this);
    } else if (parent is MethodInvocation) {
      MethodInvocation invocation = parent;
      return identical(invocation.methodName, this) &&
          invocation.realTarget != null;
    }
    return false;
  }

  @override
  bool get isSynthetic => token.isSynthetic;

  @override
  String get name => token.lexeme;

  @override
  int get precedence => 16;

  @deprecated
  @override
  Element get propagatedElement => null;

  @deprecated
  @override
  void set propagatedElement(Element element) {}

  @override
  Element get staticElement => _staticElement;

  @override
  void set staticElement(Element element) {
    _staticElement = element;
  }

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitSimpleIdentifier(this);

  @override
  bool inDeclarationContext() => false;

  @override
  bool inGetterContext() {
    // TODO(brianwilkerson) Convert this to a getter.
    AstNode initialParent = this.parent;
    AstNode parent = initialParent;
    AstNode target = this;
    // skip prefix
    if (initialParent is PrefixedIdentifier) {
      if (identical(initialParent.prefix, this)) {
        return true;
      }
      parent = initialParent.parent;
      target = initialParent;
    } else if (initialParent is PropertyAccess) {
      if (identical(initialParent.target, this)) {
        return true;
      }
      parent = initialParent.parent;
      target = initialParent;
    }
    // skip label
    if (parent is Label) {
      return false;
    }
    // analyze usage
    if (parent is AssignmentExpression) {
      if (identical(parent.leftHandSide, target) &&
          parent.operator.type == TokenType.EQ) {
        return false;
      }
    }
    if (parent is ConstructorFieldInitializer &&
        identical(parent.fieldName, target)) {
      return false;
    }
    if (parent is ForEachStatement) {
      if (identical(parent.identifier, target)) {
        return false;
      }
    }
    if (parent is FieldFormalParameter) {
      if (identical(parent.identifier, target)) {
        return false;
      }
    }
    if (parent is VariableDeclaration) {
      if (identical(parent.name, target)) {
        return false;
      }
    }
    return true;
  }

  @override
  bool inSetterContext() {
    // TODO(brianwilkerson) Convert this to a getter.
    AstNode initialParent = this.parent;
    AstNode parent = initialParent;
    AstNode target = this;
    // skip prefix
    if (initialParent is PrefixedIdentifier) {
      // if this is the prefix, then return false
      if (identical(initialParent.prefix, this)) {
        return false;
      }
      parent = initialParent.parent;
      target = initialParent;
    } else if (initialParent is PropertyAccess) {
      if (identical(initialParent.target, this)) {
        return false;
      }
      parent = initialParent.parent;
      target = initialParent;
    }
    // analyze usage
    if (parent is PrefixExpression) {
      return parent.operator.type.isIncrementOperator;
    } else if (parent is PostfixExpression) {
      return true;
    } else if (parent is AssignmentExpression) {
      return identical(parent.leftHandSide, target);
    } else if (parent is ForEachStatement) {
      return identical(parent.identifier, target);
    }
    return false;
  }

  @override
  void visitChildren(AstVisitor visitor) {
    // There are no children to visit.
  }
}

/**
 * A string literal expression that does not contain any interpolations.
 *
 *    simpleStringLiteral ::=
 *        rawStringLiteral
 *      | basicStringLiteral
 *
 *    rawStringLiteral ::=
 *        'r' basicStringLiteral
 *
 *    simpleStringLiteral ::=
 *        multiLineStringLiteral
 *      | singleLineStringLiteral
 *
 *    multiLineStringLiteral ::=
 *        "'''" characters "'''"
 *      | '"""' characters '"""'
 *
 *    singleLineStringLiteral ::=
 *        "'" characters "'"
 *      | '"' characters '"'
 */
class SimpleStringLiteralImpl extends SingleStringLiteralImpl
    implements SimpleStringLiteral {
  /**
   * The token representing the literal.
   */
  Token literal;

  /**
   * The value of the literal.
   */
  String _value;

  /**
   * Initialize a newly created simple string literal.
   */
  SimpleStringLiteralImpl(this.literal, String value) {
    _value = StringUtilities.intern(value);
  }

  @override
  Token get beginToken => literal;

  @override
  Iterable<SyntacticEntity> get childEntities =>
      new ChildEntities()..add(literal);

  @override
  int get contentsEnd => offset + _helper.end;

  @override
  int get contentsOffset => offset + _helper.start;

  @override
  Token get endToken => literal;

  @override
  bool get isMultiline => _helper.isMultiline;

  @override
  bool get isRaw => _helper.isRaw;

  @override
  bool get isSingleQuoted => _helper.isSingleQuoted;

  @override
  bool get isSynthetic => literal.isSynthetic;

  @override
  String get value => _value;

  @override
  void set value(String string) {
    _value = StringUtilities.intern(_value);
  }

  StringLexemeHelper get _helper {
    return new StringLexemeHelper(literal.lexeme, true, true);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitSimpleStringLiteral(this);

  @override
  void visitChildren(AstVisitor visitor) {
    // There are no children to visit.
  }

  @override
  void _appendStringValue(StringBuffer buffer) {
    buffer.write(value);
  }
}

/**
 * A single string literal expression.
 *
 *    singleStringLiteral ::=
 *        [SimpleStringLiteral]
 *      | [StringInterpolation]
 */
abstract class SingleStringLiteralImpl extends StringLiteralImpl
    implements SingleStringLiteral {}

class SpreadElementImpl extends AstNodeImpl
    implements CollectionElementImpl, MapElementImpl, SpreadElement {
  Token spreadOperator;

  ExpressionImpl _expression;

  SpreadElementImpl(this.spreadOperator, ExpressionImpl expression) {
    _expression = _becomeParentOf(expression);
  }

  @override
  Token get beginToken => spreadOperator;

  @override
  Iterable<SyntacticEntity> get childEntities =>
      new ChildEntities()..add(spreadOperator)..add(_expression);

  @override
  Token get endToken => _expression.endToken;

  @override
  Expression get expression => _expression;

  set expression(Expression expression) {
    _expression = _becomeParentOf(expression as ExpressionImpl);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) {
    return visitor.visitSpreadElement(this);
  }

  @override
  void visitChildren(AstVisitor visitor) {
    _expression?.accept(visitor);
  }
}

/**
 * A node that represents a statement.
 *
 *    statement ::=
 *        [Block]
 *      | [VariableDeclarationStatement]
 *      | [ForStatement]
 *      | [ForEachStatement]
 *      | [WhileStatement]
 *      | [DoStatement]
 *      | [SwitchStatement]
 *      | [IfStatement]
 *      | [TryStatement]
 *      | [BreakStatement]
 *      | [ContinueStatement]
 *      | [ReturnStatement]
 *      | [ExpressionStatement]
 *      | [FunctionDeclarationStatement]
 */
abstract class StatementImpl extends AstNodeImpl implements Statement {
  @override
  Statement get unlabeled => this;
}

/**
 * A string interpolation literal.
 *
 *    stringInterpolation ::=
 *        ''' [InterpolationElement]* '''
 *      | '"' [InterpolationElement]* '"'
 */
class StringInterpolationImpl extends SingleStringLiteralImpl
    implements StringInterpolation {
  /**
   * The elements that will be composed to produce the resulting string.
   */
  NodeList<InterpolationElement> _elements;

  /**
   * Initialize a newly created string interpolation expression.
   */
  StringInterpolationImpl(List<InterpolationElement> elements) {
    _elements = new NodeListImpl<InterpolationElement>(this, elements);
  }

  @override
  Token get beginToken => _elements.beginToken;

  @override
  Iterable<SyntacticEntity> get childEntities =>
      new ChildEntities()..addAll(_elements);

  @override
  int get contentsEnd {
    InterpolationString element = _elements.last;
    return element.contentsEnd;
  }

  @override
  int get contentsOffset {
    InterpolationString element = _elements.first;
    return element.contentsOffset;
  }

  /**
   * Return the elements that will be composed to produce the resulting string.
   */
  NodeList<InterpolationElement> get elements => _elements;

  @override
  Token get endToken => _elements.endToken;

  @override
  bool get isMultiline => _firstHelper.isMultiline;

  @override
  bool get isRaw => false;

  @override
  bool get isSingleQuoted => _firstHelper.isSingleQuoted;

  StringLexemeHelper get _firstHelper {
    InterpolationString lastString = _elements.first;
    String lexeme = lastString.contents.lexeme;
    return new StringLexemeHelper(lexeme, true, false);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitStringInterpolation(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _elements.accept(visitor);
  }

  @override
  void _appendStringValue(StringBuffer buffer) {
    throw new ArgumentError();
  }
}

/**
 * A helper for analyzing string lexemes.
 */
class StringLexemeHelper {
  final String lexeme;
  final bool isFirst;
  final bool isLast;

  bool isRaw = false;
  bool isSingleQuoted = false;
  bool isMultiline = false;
  int start = 0;
  int end;

  StringLexemeHelper(this.lexeme, this.isFirst, this.isLast) {
    if (isFirst) {
      isRaw = StringUtilities.startsWithChar(lexeme, 0x72);
      if (isRaw) {
        start++;
      }
      if (StringUtilities.startsWith3(lexeme, start, 0x27, 0x27, 0x27)) {
        isSingleQuoted = true;
        isMultiline = true;
        start += 3;
        start = _trimInitialWhitespace(start);
      } else if (StringUtilities.startsWith3(lexeme, start, 0x22, 0x22, 0x22)) {
        isSingleQuoted = false;
        isMultiline = true;
        start += 3;
        start = _trimInitialWhitespace(start);
      } else if (start < lexeme.length && lexeme.codeUnitAt(start) == 0x27) {
        isSingleQuoted = true;
        isMultiline = false;
        start++;
      } else if (start < lexeme.length && lexeme.codeUnitAt(start) == 0x22) {
        isSingleQuoted = false;
        isMultiline = false;
        start++;
      }
    }
    end = lexeme.length;
    if (isLast) {
      if (start + 3 <= end &&
          (StringUtilities.endsWith3(lexeme, 0x22, 0x22, 0x22) ||
              StringUtilities.endsWith3(lexeme, 0x27, 0x27, 0x27))) {
        end -= 3;
      } else if (start + 1 <= end &&
          (StringUtilities.endsWithChar(lexeme, 0x22) ||
              StringUtilities.endsWithChar(lexeme, 0x27))) {
        end -= 1;
      }
    }
  }

  /**
   * Given the [lexeme] for a multi-line string whose content begins at the
   * given [start] index, return the index of the first character that is
   * included in the value of the string. According to the specification:
   *
   * If the first line of a multiline string consists solely of the whitespace
   * characters defined by the production WHITESPACE 20.1), possibly prefixed
   * by \, then that line is ignored, including the new line at its end.
   */
  int _trimInitialWhitespace(int start) {
    int length = lexeme.length;
    int index = start;
    while (index < length) {
      int currentChar = lexeme.codeUnitAt(index);
      if (currentChar == 0x0D) {
        if (index + 1 < length && lexeme.codeUnitAt(index + 1) == 0x0A) {
          return index + 2;
        }
        return index + 1;
      } else if (currentChar == 0x0A) {
        return index + 1;
      } else if (currentChar == 0x5C) {
        if (index + 1 >= length) {
          return start;
        }
        currentChar = lexeme.codeUnitAt(index + 1);
        if (currentChar != 0x0D &&
            currentChar != 0x0A &&
            currentChar != 0x09 &&
            currentChar != 0x20) {
          return start;
        }
      } else if (currentChar != 0x09 && currentChar != 0x20) {
        return start;
      }
      index++;
    }
    return start;
  }
}

/**
 * A string literal expression.
 *
 *    stringLiteral ::=
 *        [SimpleStringLiteral]
 *      | [AdjacentStrings]
 *      | [StringInterpolation]
 */
abstract class StringLiteralImpl extends LiteralImpl implements StringLiteral {
  @override
  String get stringValue {
    StringBuffer buffer = new StringBuffer();
    try {
      _appendStringValue(buffer);
    } on ArgumentError {
      return null;
    }
    return buffer.toString();
  }

  /**
   * Append the value of this string literal to the given [buffer]. Throw an
   * [ArgumentError] if the string is not a constant string without any
   * string interpolation.
   */
  void _appendStringValue(StringBuffer buffer);
}

/**
 * The invocation of a superclass' constructor from within a constructor's
 * initialization list.
 *
 *    superInvocation ::=
 *        'super' ('.' [SimpleIdentifier])? [ArgumentList]
 */
class SuperConstructorInvocationImpl extends ConstructorInitializerImpl
    implements SuperConstructorInvocation {
  /**
   * The token for the 'super' keyword.
   */
  Token superKeyword;

  /**
   * The token for the period before the name of the constructor that is being
   * invoked, or `null` if the unnamed constructor is being invoked.
   */
  Token period;

  /**
   * The name of the constructor that is being invoked, or `null` if the unnamed
   * constructor is being invoked.
   */
  SimpleIdentifierImpl _constructorName;

  /**
   * The list of arguments to the constructor.
   */
  ArgumentListImpl _argumentList;

  /**
   * The element associated with the constructor based on static type
   * information, or `null` if the AST structure has not been resolved or if the
   * constructor could not be resolved.
   */
  ConstructorElement staticElement;

  /**
   * Initialize a newly created super invocation to invoke the inherited
   * constructor with the given name with the given arguments. The [period] and
   * [constructorName] can be `null` if the constructor being invoked is the
   * unnamed constructor.
   */
  SuperConstructorInvocationImpl(this.superKeyword, this.period,
      SimpleIdentifierImpl constructorName, ArgumentListImpl argumentList) {
    _constructorName = _becomeParentOf(constructorName);
    _argumentList = _becomeParentOf(argumentList);
  }

  @override
  ArgumentList get argumentList => _argumentList;

  @override
  void set argumentList(ArgumentList argumentList) {
    _argumentList = _becomeParentOf(argumentList as ArgumentListImpl);
  }

  @override
  Token get beginToken => superKeyword;

  @override
  Iterable<SyntacticEntity> get childEntities => new ChildEntities()
    ..add(superKeyword)
    ..add(period)
    ..add(_constructorName)
    ..add(_argumentList);

  @override
  SimpleIdentifier get constructorName => _constructorName;

  @override
  void set constructorName(SimpleIdentifier identifier) {
    _constructorName = _becomeParentOf(identifier as SimpleIdentifierImpl);
  }

  @override
  Token get endToken => _argumentList.endToken;

  @override
  E accept<E>(AstVisitor<E> visitor) =>
      visitor.visitSuperConstructorInvocation(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _constructorName?.accept(visitor);
    _argumentList?.accept(visitor);
  }
}

/**
 * A super expression.
 *
 *    superExpression ::=
 *        'super'
 */
class SuperExpressionImpl extends ExpressionImpl implements SuperExpression {
  /**
   * The token representing the 'super' keyword.
   */
  Token superKeyword;

  /**
   * Initialize a newly created super expression.
   */
  SuperExpressionImpl(this.superKeyword);

  @override
  Token get beginToken => superKeyword;

  @override
  Iterable<SyntacticEntity> get childEntities =>
      new ChildEntities()..add(superKeyword);

  @override
  Token get endToken => superKeyword;

  @override
  int get precedence => 16;

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitSuperExpression(this);

  @override
  void visitChildren(AstVisitor visitor) {
    // There are no children to visit.
  }
}

/**
 * A case in a switch statement.
 *
 *    switchCase ::=
 *        [SimpleIdentifier]* 'case' [Expression] ':' [Statement]*
 */
class SwitchCaseImpl extends SwitchMemberImpl implements SwitchCase {
  /**
   * The expression controlling whether the statements will be executed.
   */
  ExpressionImpl _expression;

  /**
   * Initialize a newly created switch case. The list of [labels] can be `null`
   * if there are no labels.
   */
  SwitchCaseImpl(List<Label> labels, Token keyword, ExpressionImpl expression,
      Token colon, List<Statement> statements)
      : super(labels, keyword, colon, statements) {
    _expression = _becomeParentOf(expression);
  }

  @override
  Iterable<SyntacticEntity> get childEntities => new ChildEntities()
    ..addAll(labels)
    ..add(keyword)
    ..add(_expression)
    ..add(colon)
    ..addAll(statements);

  @override
  Expression get expression => _expression;

  @override
  void set expression(Expression expression) {
    _expression = _becomeParentOf(expression as ExpressionImpl);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitSwitchCase(this);

  @override
  void visitChildren(AstVisitor visitor) {
    labels.accept(visitor);
    _expression?.accept(visitor);
    statements.accept(visitor);
  }
}

/**
 * The default case in a switch statement.
 *
 *    switchDefault ::=
 *        [SimpleIdentifier]* 'default' ':' [Statement]*
 */
class SwitchDefaultImpl extends SwitchMemberImpl implements SwitchDefault {
  /**
   * Initialize a newly created switch default. The list of [labels] can be
   * `null` if there are no labels.
   */
  SwitchDefaultImpl(List<Label> labels, Token keyword, Token colon,
      List<Statement> statements)
      : super(labels, keyword, colon, statements);

  @override
  Iterable<SyntacticEntity> get childEntities => new ChildEntities()
    ..addAll(labels)
    ..add(keyword)
    ..add(colon)
    ..addAll(statements);

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitSwitchDefault(this);

  @override
  void visitChildren(AstVisitor visitor) {
    labels.accept(visitor);
    statements.accept(visitor);
  }
}

/**
 * An element within a switch statement.
 *
 *    switchMember ::=
 *        switchCase
 *      | switchDefault
 */
abstract class SwitchMemberImpl extends AstNodeImpl implements SwitchMember {
  /**
   * The labels associated with the switch member.
   */
  NodeList<Label> _labels;

  /**
   * The token representing the 'case' or 'default' keyword.
   */
  Token keyword;

  /**
   * The colon separating the keyword or the expression from the statements.
   */
  Token colon;

  /**
   * The statements that will be executed if this switch member is selected.
   */
  NodeList<Statement> _statements;

  /**
   * Initialize a newly created switch member. The list of [labels] can be
   * `null` if there are no labels.
   */
  SwitchMemberImpl(List<Label> labels, this.keyword, this.colon,
      List<Statement> statements) {
    _labels = new NodeListImpl<Label>(this, labels);
    _statements = new NodeListImpl<Statement>(this, statements);
  }

  @override
  Token get beginToken {
    if (!_labels.isEmpty) {
      return _labels.beginToken;
    }
    return keyword;
  }

  @override
  Token get endToken {
    if (!_statements.isEmpty) {
      return _statements.endToken;
    }
    return colon;
  }

  @override
  NodeList<Label> get labels => _labels;

  @override
  NodeList<Statement> get statements => _statements;
}

/**
 * A switch statement.
 *
 *    switchStatement ::=
 *        'switch' '(' [Expression] ')' '{' [SwitchCase]* [SwitchDefault]? '}'
 */
class SwitchStatementImpl extends StatementImpl implements SwitchStatement {
  /**
   * The token representing the 'switch' keyword.
   */
  Token switchKeyword;

  /**
   * The left parenthesis.
   */
  Token leftParenthesis;

  /**
   * The expression used to determine which of the switch members will be
   * selected.
   */
  ExpressionImpl _expression;

  /**
   * The right parenthesis.
   */
  Token rightParenthesis;

  /**
   * The left curly bracket.
   */
  Token leftBracket;

  /**
   * The switch members that can be selected by the expression.
   */
  NodeList<SwitchMember> _members;

  /**
   * The right curly bracket.
   */
  Token rightBracket;

  /**
   * Initialize a newly created switch statement. The list of [members] can be
   * `null` if there are no switch members.
   */
  SwitchStatementImpl(
      this.switchKeyword,
      this.leftParenthesis,
      ExpressionImpl expression,
      this.rightParenthesis,
      this.leftBracket,
      List<SwitchMember> members,
      this.rightBracket) {
    _expression = _becomeParentOf(expression);
    _members = new NodeListImpl<SwitchMember>(this, members);
  }

  @override
  Token get beginToken => switchKeyword;

  @override
  Iterable<SyntacticEntity> get childEntities => new ChildEntities()
    ..add(switchKeyword)
    ..add(leftParenthesis)
    ..add(_expression)
    ..add(rightParenthesis)
    ..add(leftBracket)
    ..addAll(_members)
    ..add(rightBracket);

  @override
  Token get endToken => rightBracket;

  @override
  Expression get expression => _expression;

  @override
  void set expression(Expression expression) {
    _expression = _becomeParentOf(expression as ExpressionImpl);
  }

  @override
  NodeList<SwitchMember> get members => _members;

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitSwitchStatement(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _expression?.accept(visitor);
    _members.accept(visitor);
  }
}

/**
 * A symbol literal expression.
 *
 *    symbolLiteral ::=
 *        '#' (operator | (identifier ('.' identifier)*))
 */
class SymbolLiteralImpl extends LiteralImpl implements SymbolLiteral {
  /**
   * The token introducing the literal.
   */
  Token poundSign;

  /**
   * The components of the literal.
   */
  final List<Token> components;

  /**
   * Initialize a newly created symbol literal.
   */
  SymbolLiteralImpl(this.poundSign, this.components);

  @override
  Token get beginToken => poundSign;

  @override
  // TODO(paulberry): add "." tokens.
  Iterable<SyntacticEntity> get childEntities => new ChildEntities()
    ..add(poundSign)
    ..addAll(components);

  @override
  Token get endToken => components[components.length - 1];

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitSymbolLiteral(this);

  @override
  void visitChildren(AstVisitor visitor) {
    // There are no children to visit.
  }
}

/**
 * A this expression.
 *
 *    thisExpression ::=
 *        'this'
 */
class ThisExpressionImpl extends ExpressionImpl implements ThisExpression {
  /**
   * The token representing the 'this' keyword.
   */
  Token thisKeyword;

  /**
   * Initialize a newly created this expression.
   */
  ThisExpressionImpl(this.thisKeyword);

  @override
  Token get beginToken => thisKeyword;

  @override
  Iterable<SyntacticEntity> get childEntities =>
      new ChildEntities()..add(thisKeyword);

  @override
  Token get endToken => thisKeyword;

  @override
  int get precedence => 16;

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitThisExpression(this);

  @override
  void visitChildren(AstVisitor visitor) {
    // There are no children to visit.
  }
}

/**
 * A throw expression.
 *
 *    throwExpression ::=
 *        'throw' [Expression]
 */
class ThrowExpressionImpl extends ExpressionImpl implements ThrowExpression {
  /**
   * The token representing the 'throw' keyword.
   */
  Token throwKeyword;

  /**
   * The expression computing the exception to be thrown.
   */
  ExpressionImpl _expression;

  /**
   * Initialize a newly created throw expression.
   */
  ThrowExpressionImpl(this.throwKeyword, ExpressionImpl expression) {
    _expression = _becomeParentOf(expression);
  }

  @override
  Token get beginToken => throwKeyword;

  @override
  Iterable<SyntacticEntity> get childEntities =>
      new ChildEntities()..add(throwKeyword)..add(_expression);

  @override
  Token get endToken {
    if (_expression != null) {
      return _expression.endToken;
    }
    return throwKeyword;
  }

  @override
  Expression get expression => _expression;

  @override
  void set expression(Expression expression) {
    _expression = _becomeParentOf(expression as ExpressionImpl);
  }

  @override
  int get precedence => 0;

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitThrowExpression(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _expression?.accept(visitor);
  }
}

/**
 * The declaration of one or more top-level variables of the same type.
 *
 *    topLevelVariableDeclaration ::=
 *        ('final' | 'const') type? staticFinalDeclarationList ';'
 *      | variableDeclaration ';'
 */
class TopLevelVariableDeclarationImpl extends CompilationUnitMemberImpl
    implements TopLevelVariableDeclaration {
  /**
   * The top-level variables being declared.
   */
  VariableDeclarationListImpl _variableList;

  /**
   * The semicolon terminating the declaration.
   */
  Token semicolon;

  /**
   * Initialize a newly created top-level variable declaration. Either or both
   * of the [comment] and [metadata] can be `null` if the variable does not have
   * the corresponding attribute.
   */
  TopLevelVariableDeclarationImpl(
      CommentImpl comment,
      List<Annotation> metadata,
      VariableDeclarationListImpl variableList,
      this.semicolon)
      : super(comment, metadata) {
    _variableList = _becomeParentOf(variableList);
  }

  @override
  Iterable<SyntacticEntity> get childEntities =>
      super._childEntities..add(_variableList)..add(semicolon);

  @override
  Element get declaredElement => null;

  @deprecated
  @override
  Element get element => null;

  @override
  Token get endToken => semicolon;

  @override
  Token get firstTokenAfterCommentAndMetadata => _variableList.beginToken;

  @override
  VariableDeclarationList get variables => _variableList;

  @override
  void set variables(VariableDeclarationList variables) {
    _variableList = _becomeParentOf(variables as VariableDeclarationListImpl);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) =>
      visitor.visitTopLevelVariableDeclaration(this);

  @override
  void visitChildren(AstVisitor visitor) {
    super.visitChildren(visitor);
    _variableList?.accept(visitor);
  }
}

/**
 * A try statement.
 *
 *    tryStatement ::=
 *        'try' [Block] ([CatchClause]+ finallyClause? | finallyClause)
 *
 *    finallyClause ::=
 *        'finally' [Block]
 */
class TryStatementImpl extends StatementImpl implements TryStatement {
  /**
   * The token representing the 'try' keyword.
   */
  Token tryKeyword;

  /**
   * The body of the statement.
   */
  BlockImpl _body;

  /**
   * The catch clauses contained in the try statement.
   */
  NodeList<CatchClause> _catchClauses;

  /**
   * The token representing the 'finally' keyword, or `null` if the statement
   * does not contain a finally clause.
   */
  Token finallyKeyword;

  /**
   * The finally block contained in the try statement, or `null` if the
   * statement does not contain a finally clause.
   */
  BlockImpl _finallyBlock;

  /**
   * Initialize a newly created try statement. The list of [catchClauses] can be
   * `null` if there are no catch clauses. The [finallyKeyword] and
   * [finallyBlock] can be `null` if there is no finally clause.
   */
  TryStatementImpl(
      this.tryKeyword,
      BlockImpl body,
      List<CatchClause> catchClauses,
      this.finallyKeyword,
      BlockImpl finallyBlock) {
    _body = _becomeParentOf(body);
    _catchClauses = new NodeListImpl<CatchClause>(this, catchClauses);
    _finallyBlock = _becomeParentOf(finallyBlock);
  }

  @override
  Token get beginToken => tryKeyword;

  @override
  Block get body => _body;

  @override
  void set body(Block block) {
    _body = _becomeParentOf(block as BlockImpl);
  }

  @override
  NodeList<CatchClause> get catchClauses => _catchClauses;

  @override
  Iterable<SyntacticEntity> get childEntities => new ChildEntities()
    ..add(tryKeyword)
    ..add(_body)
    ..addAll(_catchClauses)
    ..add(finallyKeyword)
    ..add(_finallyBlock);

  @override
  Token get endToken {
    if (_finallyBlock != null) {
      return _finallyBlock.endToken;
    } else if (finallyKeyword != null) {
      return finallyKeyword;
    } else if (!_catchClauses.isEmpty) {
      return _catchClauses.endToken;
    }
    return _body.endToken;
  }

  @override
  Block get finallyBlock => _finallyBlock;

  @override
  void set finallyBlock(Block block) {
    _finallyBlock = _becomeParentOf(block as BlockImpl);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitTryStatement(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _body?.accept(visitor);
    _catchClauses.accept(visitor);
    _finallyBlock?.accept(visitor);
  }
}

/**
 * The declaration of a type alias.
 *
 *    typeAlias ::=
 *        'typedef' typeAliasBody
 *
 *    typeAliasBody ::=
 *        classTypeAlias
 *      | functionTypeAlias
 */
abstract class TypeAliasImpl extends NamedCompilationUnitMemberImpl
    implements TypeAlias {
  /**
   * The token representing the 'typedef' keyword.
   */
  Token typedefKeyword;

  /**
   * The semicolon terminating the declaration.
   */
  Token semicolon;

  /**
   * Initialize a newly created type alias. Either or both of the [comment] and
   * [metadata] can be `null` if the declaration does not have the corresponding
   * attribute.
   */
  TypeAliasImpl(CommentImpl comment, List<Annotation> metadata,
      this.typedefKeyword, SimpleIdentifierImpl name, this.semicolon)
      : super(comment, metadata, name);

  @override
  Token get endToken => semicolon;

  @override
  Token get firstTokenAfterCommentAndMetadata => typedefKeyword;
}

/**
 * A type annotation.
 *
 *    type ::=
 *        [NamedType]
 *      | [GenericFunctionType]
 */
abstract class TypeAnnotationImpl extends AstNodeImpl
    implements TypeAnnotation {}

/**
 * A list of type arguments.
 *
 *    typeArguments ::=
 *        '<' typeName (',' typeName)* '>'
 */
class TypeArgumentListImpl extends AstNodeImpl implements TypeArgumentList {
  /**
   * The left bracket.
   */
  Token leftBracket;

  /**
   * The type arguments associated with the type.
   */
  NodeList<TypeAnnotation> _arguments;

  /**
   * The right bracket.
   */
  Token rightBracket;

  /**
   * Initialize a newly created list of type arguments.
   */
  TypeArgumentListImpl(
      this.leftBracket, List<TypeAnnotation> arguments, this.rightBracket) {
    _arguments = new NodeListImpl<TypeAnnotation>(this, arguments);
  }

  @override
  NodeList<TypeAnnotation> get arguments => _arguments;

  @override
  Token get beginToken => leftBracket;

  @override
  // TODO(paulberry): Add commas.
  Iterable<SyntacticEntity> get childEntities => new ChildEntities()
    ..add(leftBracket)
    ..addAll(_arguments)
    ..add(rightBracket);

  @override
  Token get endToken => rightBracket;

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitTypeArgumentList(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _arguments.accept(visitor);
  }
}

/**
 * A literal that has a type associated with it.
 *
 *    typedLiteral ::=
 *        [ListLiteral]
 *      | [MapLiteral]
 */
abstract class TypedLiteralImpl extends LiteralImpl implements TypedLiteral {
  /**
   * The token representing the 'const' keyword, or `null` if the literal is not
   * a constant.
   */
  Token constKeyword;

  /**
   * The type argument associated with this literal, or `null` if no type
   * arguments were declared.
   */
  TypeArgumentListImpl _typeArguments;

  /**
   * Initialize a newly created typed literal. The [constKeyword] can be `null`\
   * if the literal is not a constant. The [typeArguments] can be `null` if no
   * type arguments were declared.
   */
  TypedLiteralImpl(this.constKeyword, TypeArgumentListImpl typeArguments) {
    _typeArguments = _becomeParentOf(typeArguments);
  }

  @override
  bool get isConst {
    return constKeyword != null || inConstantContext;
  }

  @override
  TypeArgumentList get typeArguments => _typeArguments;

  @override
  void set typeArguments(TypeArgumentList typeArguments) {
    _typeArguments = _becomeParentOf(typeArguments as TypeArgumentListImpl);
  }

  ChildEntities get _childEntities =>
      new ChildEntities()..add(constKeyword)..add(_typeArguments);

  @override
  void visitChildren(AstVisitor visitor) {
    _typeArguments?.accept(visitor);
  }
}

/**
 * The name of a type, which can optionally include type arguments.
 *
 *    typeName ::=
 *        [Identifier] typeArguments? '?'?
 */
class TypeNameImpl extends TypeAnnotationImpl implements TypeName {
  /**
   * The name of the type.
   */
  IdentifierImpl _name;

  /**
   * The type arguments associated with the type, or `null` if there are no type
   * arguments.
   */
  TypeArgumentListImpl _typeArguments;

  @override
  Token question;

  /**
   * The type being named, or `null` if the AST structure has not been resolved.
   */
  DartType type;

  /**
   * Initialize a newly created type name. The [typeArguments] can be `null` if
   * there are no type arguments.
   */
  TypeNameImpl(IdentifierImpl name, TypeArgumentListImpl typeArguments,
      {this.question}) {
    _name = _becomeParentOf(name);
    _typeArguments = _becomeParentOf(typeArguments);
  }

  @override
  Token get beginToken => _name.beginToken;

  @override
  Iterable<SyntacticEntity> get childEntities =>
      new ChildEntities()..add(_name)..add(_typeArguments)..add(question);

  @override
  Token get endToken => question ?? _typeArguments?.endToken ?? _name.endToken;

  @override
  bool get isDeferred {
    Identifier identifier = name;
    if (identifier is! PrefixedIdentifier) {
      return false;
    }
    return (identifier as PrefixedIdentifier).isDeferred;
  }

  @override
  bool get isSynthetic => _name.isSynthetic && _typeArguments == null;

  @override
  Identifier get name => _name;

  @override
  void set name(Identifier identifier) {
    _name = _becomeParentOf(identifier as IdentifierImpl);
  }

  @override
  TypeArgumentList get typeArguments => _typeArguments;

  @override
  void set typeArguments(TypeArgumentList typeArguments) {
    _typeArguments = _becomeParentOf(typeArguments as TypeArgumentListImpl);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitTypeName(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _name?.accept(visitor);
    _typeArguments?.accept(visitor);
  }
}

/**
 * A type parameter.
 *
 *    typeParameter ::=
 *        [SimpleIdentifier] ('extends' [TypeName])?
 */
class TypeParameterImpl extends DeclarationImpl implements TypeParameter {
  /**
   * The name of the type parameter.
   */
  SimpleIdentifierImpl _name;

  /**
   * The token representing the 'extends' keyword, or `null` if there is no
   * explicit upper bound.
   */
  Token extendsKeyword;

  /**
   * The name of the upper bound for legal arguments, or `null` if there is no
   * explicit upper bound.
   */
  TypeAnnotationImpl _bound;

  /**
   * Initialize a newly created type parameter. Either or both of the [comment]
   * and [metadata] can be `null` if the parameter does not have the
   * corresponding attribute. The [extendsKeyword] and [bound] can be `null` if
   * the parameter does not have an upper bound.
   */
  TypeParameterImpl(CommentImpl comment, List<Annotation> metadata,
      SimpleIdentifierImpl name, this.extendsKeyword, TypeAnnotationImpl bound)
      : super(comment, metadata) {
    _name = _becomeParentOf(name);
    _bound = _becomeParentOf(bound);
  }

  @override
  TypeAnnotation get bound => _bound;

  @override
  void set bound(TypeAnnotation type) {
    _bound = _becomeParentOf(type as TypeAnnotationImpl);
  }

  @override
  Iterable<SyntacticEntity> get childEntities =>
      super._childEntities..add(_name)..add(extendsKeyword)..add(_bound);

  @override
  TypeParameterElement get declaredElement =>
      _name?.staticElement as TypeParameterElement;

  @deprecated
  @override
  TypeParameterElement get element => declaredElement;

  @override
  Token get endToken {
    if (_bound == null) {
      return _name.endToken;
    }
    return _bound.endToken;
  }

  @override
  Token get firstTokenAfterCommentAndMetadata => _name.beginToken;

  @override
  SimpleIdentifier get name => _name;

  @override
  void set name(SimpleIdentifier identifier) {
    _name = _becomeParentOf(identifier as SimpleIdentifierImpl);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitTypeParameter(this);

  @override
  void visitChildren(AstVisitor visitor) {
    super.visitChildren(visitor);
    _name?.accept(visitor);
    _bound?.accept(visitor);
  }
}

/**
 * Type parameters within a declaration.
 *
 *    typeParameterList ::=
 *        '<' [TypeParameter] (',' [TypeParameter])* '>'
 */
class TypeParameterListImpl extends AstNodeImpl implements TypeParameterList {
  /**
   * The left angle bracket.
   */
  final Token leftBracket;

  /**
   * The type parameters in the list.
   */
  NodeList<TypeParameter> _typeParameters;

  /**
   * The right angle bracket.
   */
  final Token rightBracket;

  /**
   * Initialize a newly created list of type parameters.
   */
  TypeParameterListImpl(
      this.leftBracket, List<TypeParameter> typeParameters, this.rightBracket) {
    _typeParameters = new NodeListImpl<TypeParameter>(this, typeParameters);
  }

  @override
  Token get beginToken => leftBracket;

  @override
  Iterable<SyntacticEntity> get childEntities => new ChildEntities()
    ..add(leftBracket)
    ..addAll(_typeParameters)
    ..add(rightBracket);

  @override
  Token get endToken => rightBracket;

  @override
  NodeList<TypeParameter> get typeParameters => _typeParameters;

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitTypeParameterList(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _typeParameters.accept(visitor);
  }
}

/**
 * A directive that references a URI.
 *
 *    uriBasedDirective ::=
 *        [ExportDirective]
 *      | [ImportDirective]
 *      | [PartDirective]
 */
abstract class UriBasedDirectiveImpl extends DirectiveImpl
    implements UriBasedDirective {
  /**
   * The prefix of a URI using the `dart-ext` scheme to reference a native code
   * library.
   */
  static String _DART_EXT_SCHEME = "dart-ext:";

  /**
   * The URI referenced by this directive.
   */
  StringLiteralImpl _uri;

  @override
  String uriContent;

  @override
  Source uriSource;

  /**
   * Initialize a newly create URI-based directive. Either or both of the
   * [comment] and [metadata] can be `null` if the directive does not have the
   * corresponding attribute.
   */
  UriBasedDirectiveImpl(
      CommentImpl comment, List<Annotation> metadata, StringLiteralImpl uri)
      : super(comment, metadata) {
    _uri = _becomeParentOf(uri);
  }

  @deprecated
  @override
  Source get source => uriSource;

  @deprecated
  @override
  void set source(Source source) {
    uriSource = source;
  }

  @override
  StringLiteral get uri => _uri;

  @override
  void set uri(StringLiteral uri) {
    _uri = _becomeParentOf(uri as StringLiteralImpl);
  }

  UriValidationCode validate() {
    return validateUri(this is ImportDirective, uri, uriContent);
  }

  @override
  void visitChildren(AstVisitor visitor) {
    super.visitChildren(visitor);
    _uri?.accept(visitor);
  }

  /**
   * Validate this directive, but do not check for existence. Return a code
   * indicating the problem if there is one, or `null` no problem.
   */
  static UriValidationCode validateUri(
      bool isImport, StringLiteral uriLiteral, String uriContent) {
    if (uriLiteral is StringInterpolation) {
      return UriValidationCode.URI_WITH_INTERPOLATION;
    }
    if (uriContent == null) {
      return UriValidationCode.INVALID_URI;
    }
    if (isImport && uriContent.startsWith(_DART_EXT_SCHEME)) {
      return UriValidationCode.URI_WITH_DART_EXT_SCHEME;
    }
    Uri uri;
    try {
      uri = Uri.parse(Uri.encodeFull(uriContent));
    } on FormatException {
      return UriValidationCode.INVALID_URI;
    }
    if (uri.path.isEmpty) {
      return UriValidationCode.INVALID_URI;
    }
    return null;
  }
}

/**
 * Validation codes returned by [UriBasedDirective.validate].
 */
class UriValidationCode {
  static const UriValidationCode INVALID_URI =
      const UriValidationCode('INVALID_URI');

  static const UriValidationCode URI_WITH_INTERPOLATION =
      const UriValidationCode('URI_WITH_INTERPOLATION');

  static const UriValidationCode URI_WITH_DART_EXT_SCHEME =
      const UriValidationCode('URI_WITH_DART_EXT_SCHEME');

  /**
   * The name of the validation code.
   */
  final String name;

  /**
   * Initialize a newly created validation code to have the given [name].
   */
  const UriValidationCode(this.name);

  @override
  String toString() => name;
}

/**
 * An identifier that has an initial value associated with it. Instances of this
 * class are always children of the class [VariableDeclarationList].
 *
 *    variableDeclaration ::=
 *        [SimpleIdentifier] ('=' [Expression])?
 *
 * TODO(paulberry): the grammar does not allow metadata to be associated with
 * a VariableDeclaration, and currently we don't record comments for it either.
 * Consider changing the class hierarchy so that [VariableDeclaration] does not
 * extend [Declaration].
 */
class VariableDeclarationImpl extends DeclarationImpl
    implements VariableDeclaration {
  /**
   * The name of the variable being declared.
   */
  SimpleIdentifierImpl _name;

  /**
   * The equal sign separating the variable name from the initial value, or
   * `null` if the initial value was not specified.
   */
  Token equals;

  /**
   * The expression used to compute the initial value for the variable, or
   * `null` if the initial value was not specified.
   */
  ExpressionImpl _initializer;

  /**
   * Initialize a newly created variable declaration. The [equals] and
   * [initializer] can be `null` if there is no initializer.
   */
  VariableDeclarationImpl(
      SimpleIdentifierImpl name, this.equals, ExpressionImpl initializer)
      : super(null, null) {
    _name = _becomeParentOf(name);
    _initializer = _becomeParentOf(initializer);
  }

  @override
  Iterable<SyntacticEntity> get childEntities =>
      super._childEntities..add(_name)..add(equals)..add(_initializer);

  @override
  VariableElement get declaredElement =>
      _name?.staticElement as VariableElement;

  /**
   * This overridden implementation of [documentationComment] looks in the
   * grandparent node for Dartdoc comments if no documentation is specifically
   * available on the node.
   */
  @override
  Comment get documentationComment {
    Comment comment = super.documentationComment;
    if (comment == null) {
      AstNode node = parent?.parent;
      if (node is AnnotatedNode) {
        return node.documentationComment;
      }
    }
    return comment;
  }

  @deprecated
  @override
  VariableElement get element => declaredElement;

  @override
  Token get endToken {
    if (_initializer != null) {
      return _initializer.endToken;
    }
    return _name.endToken;
  }

  @override
  Token get firstTokenAfterCommentAndMetadata => _name.beginToken;

  @override
  Expression get initializer => _initializer;

  @override
  void set initializer(Expression expression) {
    _initializer = _becomeParentOf(expression as ExpressionImpl);
  }

  @override
  bool get isConst {
    AstNode parent = this.parent;
    return parent is VariableDeclarationList && parent.isConst;
  }

  @override
  bool get isFinal {
    AstNode parent = this.parent;
    return parent is VariableDeclarationList && parent.isFinal;
  }

  @override
  SimpleIdentifier get name => _name;

  @override
  void set name(SimpleIdentifier identifier) {
    _name = _becomeParentOf(identifier as SimpleIdentifierImpl);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitVariableDeclaration(this);

  @override
  void visitChildren(AstVisitor visitor) {
    super.visitChildren(visitor);
    _name?.accept(visitor);
    _initializer?.accept(visitor);
  }
}

/**
 * The declaration of one or more variables of the same type.
 *
 *    variableDeclarationList ::=
 *        finalConstVarOrType [VariableDeclaration] (',' [VariableDeclaration])*
 *
 *    finalConstVarOrType ::=
 *      | 'final' [TypeName]?
 *      | 'const' [TypeName]?
 *      | 'var'
 *      | [TypeName]
 */
class VariableDeclarationListImpl extends AnnotatedNodeImpl
    implements VariableDeclarationList {
  /**
   * The token representing the 'final', 'const' or 'var' keyword, or `null` if
   * no keyword was included.
   */
  Token keyword;

  /**
   * The type of the variables being declared, or `null` if no type was provided.
   */
  TypeAnnotationImpl _type;

  /**
   * A list containing the individual variables being declared.
   */
  NodeList<VariableDeclaration> _variables;

  /**
   * Initialize a newly created variable declaration list. Either or both of the
   * [comment] and [metadata] can be `null` if the variable list does not have
   * the corresponding attribute. The [keyword] can be `null` if a type was
   * specified. The [type] must be `null` if the keyword is 'var'.
   */
  VariableDeclarationListImpl(
      CommentImpl comment,
      List<Annotation> metadata,
      this.keyword,
      TypeAnnotationImpl type,
      List<VariableDeclaration> variables)
      : super(comment, metadata) {
    _type = _becomeParentOf(type);
    _variables = new NodeListImpl<VariableDeclaration>(this, variables);
  }

  @override
  // TODO(paulberry): include commas.
  Iterable<SyntacticEntity> get childEntities => super._childEntities
    ..add(keyword)
    ..add(_type)
    ..addAll(_variables);

  @override
  Token get endToken => _variables.endToken;

  @override
  Token get firstTokenAfterCommentAndMetadata {
    if (keyword != null) {
      return keyword;
    } else if (_type != null) {
      return _type.beginToken;
    }
    return _variables.beginToken;
  }

  @override
  bool get isConst => keyword?.keyword == Keyword.CONST;

  @override
  bool get isFinal => keyword?.keyword == Keyword.FINAL;

  @override
  TypeAnnotation get type => _type;

  @override
  void set type(TypeAnnotation type) {
    _type = _becomeParentOf(type as TypeAnnotationImpl);
  }

  @override
  NodeList<VariableDeclaration> get variables => _variables;

  @override
  E accept<E>(AstVisitor<E> visitor) =>
      visitor.visitVariableDeclarationList(this);

  @override
  void visitChildren(AstVisitor visitor) {
    super.visitChildren(visitor);
    _type?.accept(visitor);
    _variables.accept(visitor);
  }
}

/**
 * A list of variables that are being declared in a context where a statement is
 * required.
 *
 *    variableDeclarationStatement ::=
 *        [VariableDeclarationList] ';'
 */
class VariableDeclarationStatementImpl extends StatementImpl
    implements VariableDeclarationStatement {
  /**
   * The variables being declared.
   */
  VariableDeclarationListImpl _variableList;

  /**
   * The semicolon terminating the statement.
   */
  Token semicolon;

  /**
   * Initialize a newly created variable declaration statement.
   */
  VariableDeclarationStatementImpl(
      VariableDeclarationListImpl variableList, this.semicolon) {
    _variableList = _becomeParentOf(variableList);
  }

  @override
  Token get beginToken => _variableList.beginToken;

  @override
  Iterable<SyntacticEntity> get childEntities =>
      new ChildEntities()..add(_variableList)..add(semicolon);

  @override
  Token get endToken => semicolon;

  @override
  VariableDeclarationList get variables => _variableList;

  @override
  void set variables(VariableDeclarationList variables) {
    _variableList = _becomeParentOf(variables as VariableDeclarationListImpl);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) =>
      visitor.visitVariableDeclarationStatement(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _variableList?.accept(visitor);
  }
}

/**
 * A while statement.
 *
 *    whileStatement ::=
 *        'while' '(' [Expression] ')' [Statement]
 */
class WhileStatementImpl extends StatementImpl implements WhileStatement {
  /**
   * The token representing the 'while' keyword.
   */
  Token whileKeyword;

  /**
   * The left parenthesis.
   */
  Token leftParenthesis;

  /**
   * The expression used to determine whether to execute the body of the loop.
   */
  ExpressionImpl _condition;

  /**
   * The right parenthesis.
   */
  Token rightParenthesis;

  /**
   * The body of the loop.
   */
  StatementImpl _body;

  /**
   * Initialize a newly created while statement.
   */
  WhileStatementImpl(this.whileKeyword, this.leftParenthesis,
      ExpressionImpl condition, this.rightParenthesis, StatementImpl body) {
    _condition = _becomeParentOf(condition);
    _body = _becomeParentOf(body);
  }

  @override
  Token get beginToken => whileKeyword;

  @override
  Statement get body => _body;

  @override
  void set body(Statement statement) {
    _body = _becomeParentOf(statement as StatementImpl);
  }

  @override
  Iterable<SyntacticEntity> get childEntities => new ChildEntities()
    ..add(whileKeyword)
    ..add(leftParenthesis)
    ..add(_condition)
    ..add(rightParenthesis)
    ..add(_body);

  @override
  Expression get condition => _condition;

  @override
  void set condition(Expression expression) {
    _condition = _becomeParentOf(expression as ExpressionImpl);
  }

  @override
  Token get endToken => _body.endToken;

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitWhileStatement(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _condition?.accept(visitor);
    _body?.accept(visitor);
  }
}

/**
 * The with clause in a class declaration.
 *
 *    withClause ::=
 *        'with' [TypeName] (',' [TypeName])*
 */
class WithClauseImpl extends AstNodeImpl implements WithClause {
  /**
   * The token representing the 'with' keyword.
   */
  Token withKeyword;

  /**
   * The names of the mixins that were specified.
   */
  NodeList<TypeName> _mixinTypes;

  /**
   * Initialize a newly created with clause.
   */
  WithClauseImpl(this.withKeyword, List<TypeName> mixinTypes) {
    _mixinTypes = new NodeListImpl<TypeName>(this, mixinTypes);
  }

  @override
  Token get beginToken => withKeyword;

  @override
  // TODO(paulberry): add commas.
  Iterable<SyntacticEntity> get childEntities => new ChildEntities()
    ..add(withKeyword)
    ..addAll(_mixinTypes);

  @override
  Token get endToken => _mixinTypes.endToken;

  @override
  NodeList<TypeName> get mixinTypes => _mixinTypes;

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitWithClause(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _mixinTypes.accept(visitor);
  }
}

/**
 * A yield statement.
 *
 *    yieldStatement ::=
 *        'yield' '*'? [Expression] ‘;’
 */
class YieldStatementImpl extends StatementImpl implements YieldStatement {
  /**
   * The 'yield' keyword.
   */
  Token yieldKeyword;

  /**
   * The star optionally following the 'yield' keyword.
   */
  Token star;

  /**
   * The expression whose value will be yielded.
   */
  ExpressionImpl _expression;

  /**
   * The semicolon following the expression.
   */
  Token semicolon;

  /**
   * Initialize a newly created yield expression. The [star] can be `null` if no
   * star was provided.
   */
  YieldStatementImpl(
      this.yieldKeyword, this.star, ExpressionImpl expression, this.semicolon) {
    _expression = _becomeParentOf(expression);
  }

  @override
  Token get beginToken {
    if (yieldKeyword != null) {
      return yieldKeyword;
    }
    return _expression.beginToken;
  }

  @override
  Iterable<SyntacticEntity> get childEntities => new ChildEntities()
    ..add(yieldKeyword)
    ..add(star)
    ..add(_expression)
    ..add(semicolon);

  @override
  Token get endToken {
    if (semicolon != null) {
      return semicolon;
    }
    return _expression.endToken;
  }

  @override
  Expression get expression => _expression;

  @override
  void set expression(Expression expression) {
    _expression = _becomeParentOf(expression as ExpressionImpl);
  }

  @override
  E accept<E>(AstVisitor<E> visitor) => visitor.visitYieldStatement(this);

  @override
  void visitChildren(AstVisitor visitor) {
    _expression?.accept(visitor);
  }
}