pkg/analyzer/lib/src/generated/resolver.dart

// 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.

library analyzer.src.generated.resolver;

import 'dart:collection';

import 'package:analyzer/dart/ast/ast.dart';
import 'package:analyzer/dart/ast/standard_resolution_map.dart';
import 'package:analyzer/dart/ast/token.dart';
import 'package:analyzer/dart/ast/visitor.dart';
import 'package:analyzer/dart/element/element.dart';
import 'package:analyzer/dart/element/type.dart';
import 'package:analyzer/dart/element/visitor.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/ast.dart';
import 'package:analyzer/src/dart/ast/utilities.dart';
import 'package:analyzer/src/dart/element/element.dart';
import 'package:analyzer/src/dart/element/member.dart' show ConstructorMember;
import 'package:analyzer/src/dart/element/type.dart';
import 'package:analyzer/src/dart/resolver/inheritance_manager.dart';
import 'package:analyzer/src/dart/resolver/scope.dart';
import 'package:analyzer/src/error/codes.dart';
import 'package:analyzer/src/generated/constant.dart';
import 'package:analyzer/src/generated/element_resolver.dart';
import 'package:analyzer/src/generated/engine.dart';
import 'package:analyzer/src/generated/error_verifier.dart';
import 'package:analyzer/src/generated/source.dart';
import 'package:analyzer/src/generated/static_type_analyzer.dart';
import 'package:analyzer/src/generated/testing/element_factory.dart';
import 'package:analyzer/src/generated/type_system.dart';
import 'package:analyzer/src/generated/utilities_dart.dart';

export 'package:analyzer/src/dart/resolver/inheritance_manager.dart';
export 'package:analyzer/src/dart/resolver/scope.dart';
export 'package:analyzer/src/generated/type_system.dart';

/**
 * Instances of the class `BestPracticesVerifier` traverse an AST structure looking for
 * violations of Dart best practices.
 */
class BestPracticesVerifier extends RecursiveAstVisitor<Object> {
//  static String _HASHCODE_GETTER_NAME = "hashCode";

  static String _NULL_TYPE_NAME = "Null";

  static String _TO_INT_METHOD_NAME = "toInt";

  /**
   * The class containing the AST nodes being visited, or `null` if we are not in the scope of
   * a class.
   */
  ClassElementImpl _enclosingClass;

  /**
   * A flag indicating whether a surrounding member (compilation unit or class)
   * is deprecated.
   */
  bool inDeprecatedMember;

  /**
   * The error reporter by which errors will be reported.
   */
  final ErrorReporter _errorReporter;

  /**
   * The type [Null].
   */
  final InterfaceType _nullType;

  /**
   * The type Future<Null>, which is needed for determining whether it is safe
   * to have a bare "return;" in an async method.
   */
  final InterfaceType _futureNullType;

  /**
   * The type system primitives
   */
  TypeSystem _typeSystem;

  /**
   * The current library
   */
  LibraryElement _currentLibrary;

  /**
   * The inheritance manager used to find overridden methods.
   */
  InheritanceManager _manager;

  /**
   * Create a new instance of the [BestPracticesVerifier].
   *
   * @param errorReporter the error reporter
   */
  BestPracticesVerifier(this._errorReporter, TypeProvider typeProvider,
      this._currentLibrary, this._manager,
      {TypeSystem typeSystem})
      : _nullType = typeProvider.nullType,
        _futureNullType = typeProvider.futureNullType,
        _typeSystem = typeSystem ?? new TypeSystemImpl(typeProvider) {
    inDeprecatedMember = _currentLibrary.isDeprecated;
  }

  @override
  Object visitAnnotation(Annotation node) {
    ElementAnnotation element =
        resolutionMap.elementAnnotationForAnnotation(node);
    if (element?.isFactory == true) {
      AstNode parent = node.parent;
      if (parent is MethodDeclaration) {
        _checkForInvalidFactory(parent);
      } else {
        _errorReporter
            .reportErrorForNode(HintCode.INVALID_FACTORY_ANNOTATION, node, []);
      }
    } else if (element?.isImmutable == true) {
      AstNode parent = node.parent;
      if (parent is! ClassDeclaration) {
        _errorReporter.reportErrorForNode(
            HintCode.INVALID_IMMUTABLE_ANNOTATION, node, []);
      }
    }
    return super.visitAnnotation(node);
  }

  @override
  Object visitArgumentList(ArgumentList node) {
    for (Expression argument in node.arguments) {
      ParameterElement parameter = argument.bestParameterElement;
      if (parameter?.parameterKind == ParameterKind.POSITIONAL) {
        _checkForDeprecatedMemberUse(parameter, argument);
      }
    }
    _checkForArgumentTypesNotAssignableInList(node);
    return super.visitArgumentList(node);
  }

  @override
  Object visitAsExpression(AsExpression node) {
    _checkForUnnecessaryCast(node);
    return super.visitAsExpression(node);
  }

  @override
  Object visitAssertInitializer(AssertInitializer node) {
    _checkForPossibleNullCondition(node.condition);
    return super.visitAssertInitializer(node);
  }

  @override
  Object visitAssertStatement(AssertStatement node) {
    _checkForPossibleNullCondition(node.condition);
    return super.visitAssertStatement(node);
  }

  @override
  Object visitAssignmentExpression(AssignmentExpression node) {
    TokenType operatorType = node.operator.type;
    if (operatorType == TokenType.EQ) {
      _checkForUseOfVoidResult(node.rightHandSide);
      _checkForInvalidAssignment(node.leftHandSide, node.rightHandSide);
    } else {
      _checkForDeprecatedMemberUse(node.bestElement, node);
    }
    return super.visitAssignmentExpression(node);
  }

  @override
  Object visitBinaryExpression(BinaryExpression node) {
    _checkForDivisionOptimizationHint(node);
    _checkForDeprecatedMemberUse(node.bestElement, node);
    return super.visitBinaryExpression(node);
  }

  @override
  Object visitClassDeclaration(ClassDeclaration node) {
    ClassElementImpl outerClass = _enclosingClass;
    bool wasInDeprecatedMember = inDeprecatedMember;
    ClassElement element = AbstractClassElementImpl.getImpl(node.element);
    if (element != null && element.isDeprecated) {
      inDeprecatedMember = true;
    }
    try {
      _enclosingClass = element;
      // Commented out until we decide that we want this hint in the analyzer
      //    checkForOverrideEqualsButNotHashCode(node);
      _checkForImmutable(node);
      return super.visitClassDeclaration(node);
    } finally {
      _enclosingClass = outerClass;
      inDeprecatedMember = wasInDeprecatedMember;
    }
  }

  @override
  Object visitConditionalExpression(ConditionalExpression node) {
    _checkForPossibleNullCondition(node.condition);
    return super.visitConditionalExpression(node);
  }

  @override
  Object visitConstructorDeclaration(ConstructorDeclaration node) {
    if (resolutionMap.elementDeclaredByConstructorDeclaration(node).isFactory) {
      if (node.body is BlockFunctionBody) {
        // Check the block for a return statement, if not, create the hint.
        if (!ExitDetector.exits(node.body)) {
          _errorReporter.reportErrorForNode(
              HintCode.MISSING_RETURN, node, [node.returnType.name]);
        }
      }
    }
    return super.visitConstructorDeclaration(node);
  }

  @override
  Object visitDoStatement(DoStatement node) {
    _checkForPossibleNullCondition(node.condition);
    return super.visitDoStatement(node);
  }

  @override
  Object visitExportDirective(ExportDirective node) {
    _checkForDeprecatedMemberUse(node.uriElement, node);
    return super.visitExportDirective(node);
  }

  @override
  Object visitForStatement(ForStatement node) {
    _checkForPossibleNullCondition(node.condition);
    return super.visitForStatement(node);
  }

  @override
  Object visitFunctionDeclaration(FunctionDeclaration node) {
    bool wasInDeprecatedMember = inDeprecatedMember;
    ExecutableElement element = node.element;
    if (element != null && element.isDeprecated) {
      inDeprecatedMember = true;
    }
    try {
      _checkForMissingReturn(node.returnType, node.functionExpression.body);
      return super.visitFunctionDeclaration(node);
    } finally {
      inDeprecatedMember = wasInDeprecatedMember;
    }
  }

  @override
  Object visitIfStatement(IfStatement node) {
    _checkForPossibleNullCondition(node.condition);
    return super.visitIfStatement(node);
  }

  @override
  Object visitImportDirective(ImportDirective node) {
    _checkForDeprecatedMemberUse(node.uriElement, node);
    ImportElement importElement = node.element;
    if (importElement != null && importElement.isDeferred) {
      _checkForLoadLibraryFunction(node, importElement);
    }
    return super.visitImportDirective(node);
  }

  @override
  Object visitIndexExpression(IndexExpression node) {
    _checkForDeprecatedMemberUse(node.bestElement, node);
    return super.visitIndexExpression(node);
  }

  @override
  Object visitInstanceCreationExpression(InstanceCreationExpression node) {
    _checkForDeprecatedMemberUse(node.staticElement, node);
    return super.visitInstanceCreationExpression(node);
  }

  @override
  Object visitIsExpression(IsExpression node) {
    _checkAllTypeChecks(node);
    return super.visitIsExpression(node);
  }

  @override
  Object visitMethodDeclaration(MethodDeclaration node) {
    bool wasInDeprecatedMember = inDeprecatedMember;
    ExecutableElement element = node.element;
    if (element != null && element.isDeprecated) {
      inDeprecatedMember = true;
    }
    try {
      // This was determined to not be a good hint, see: dartbug.com/16029
      //checkForOverridingPrivateMember(node);
      _checkForMissingReturn(node.returnType, node.body);
      _checkForUnnecessaryNoSuchMethod(node);
      return super.visitMethodDeclaration(node);
    } finally {
      inDeprecatedMember = wasInDeprecatedMember;
    }
  }

  @override
  Object visitMethodInvocation(MethodInvocation node) {
    Expression realTarget = node.realTarget;
    _checkForAbstractSuperMemberReference(realTarget, node.methodName);
    _checkForCanBeNullAfterNullAware(
        realTarget, node.operator, null, node.methodName);
    DartType staticInvokeType = node.staticInvokeType;
    if (staticInvokeType is InterfaceType) {
      MethodElement methodElement = staticInvokeType.lookUpMethod(
          FunctionElement.CALL_METHOD_NAME, _currentLibrary);
      _checkForDeprecatedMemberUse(methodElement, node);
    }
    return super.visitMethodInvocation(node);
  }

  @override
  Object visitPostfixExpression(PostfixExpression node) {
    _checkForDeprecatedMemberUse(node.bestElement, node);
    return super.visitPostfixExpression(node);
  }

  @override
  Object visitPrefixExpression(PrefixExpression node) {
    _checkForDeprecatedMemberUse(node.bestElement, node);
    return super.visitPrefixExpression(node);
  }

  @override
  Object visitPropertyAccess(PropertyAccess node) {
    Expression realTarget = node.realTarget;
    _checkForAbstractSuperMemberReference(realTarget, node.propertyName);
    _checkForCanBeNullAfterNullAware(
        realTarget, node.operator, node.propertyName, null);
    return super.visitPropertyAccess(node);
  }

  @override
  Object visitRedirectingConstructorInvocation(
      RedirectingConstructorInvocation node) {
    _checkForDeprecatedMemberUse(node.staticElement, node);
    return super.visitRedirectingConstructorInvocation(node);
  }

  @override
  Object visitSimpleIdentifier(SimpleIdentifier node) {
    _checkForDeprecatedMemberUseAtIdentifier(node);
    _checkForInvalidProtectedMemberAccess(node);
    return super.visitSimpleIdentifier(node);
  }

  @override
  Object visitSuperConstructorInvocation(SuperConstructorInvocation node) {
    _checkForDeprecatedMemberUse(node.staticElement, node);
    return super.visitSuperConstructorInvocation(node);
  }

  @override
  Object visitVariableDeclaration(VariableDeclaration node) {
    _checkForUseOfVoidResult(node.initializer);
    _checkForInvalidAssignment(node.name, node.initializer);
    return super.visitVariableDeclaration(node);
  }

  @override
  Object visitWhileStatement(WhileStatement node) {
    _checkForPossibleNullCondition(node.condition);
    return super.visitWhileStatement(node);
  }

  /**
   * Check for the passed is expression for the unnecessary type check hint codes as well as null
   * checks expressed using an is expression.
   *
   * @param node the is expression to check
   * @return `true` if and only if a hint code is generated on the passed node
   * See [HintCode.TYPE_CHECK_IS_NOT_NULL], [HintCode.TYPE_CHECK_IS_NULL],
   * [HintCode.UNNECESSARY_TYPE_CHECK_TRUE], and
   * [HintCode.UNNECESSARY_TYPE_CHECK_FALSE].
   */
  bool _checkAllTypeChecks(IsExpression node) {
    Expression expression = node.expression;
    TypeAnnotation typeName = node.type;
    DartType lhsType = expression.staticType;
    DartType rhsType = typeName.type;
    if (lhsType == null || rhsType == null) {
      return false;
    }
    String rhsNameStr = typeName is TypeName ? typeName.name.name : null;
    // if x is dynamic
    if (rhsType.isDynamic && rhsNameStr == Keyword.DYNAMIC.syntax) {
      if (node.notOperator == null) {
        // the is case
        _errorReporter.reportErrorForNode(
            HintCode.UNNECESSARY_TYPE_CHECK_TRUE, node);
      } else {
        // the is not case
        _errorReporter.reportErrorForNode(
            HintCode.UNNECESSARY_TYPE_CHECK_FALSE, node);
      }
      return true;
    }
    Element rhsElement = rhsType.element;
    LibraryElement libraryElement = rhsElement?.library;
    if (libraryElement != null && libraryElement.isDartCore) {
      // if x is Object or null is Null
      if (rhsType.isObject ||
          (expression is NullLiteral && rhsNameStr == _NULL_TYPE_NAME)) {
        if (node.notOperator == null) {
          // the is case
          _errorReporter.reportErrorForNode(
              HintCode.UNNECESSARY_TYPE_CHECK_TRUE, node);
        } else {
          // the is not case
          _errorReporter.reportErrorForNode(
              HintCode.UNNECESSARY_TYPE_CHECK_FALSE, node);
        }
        return true;
      } else if (rhsNameStr == _NULL_TYPE_NAME) {
        if (node.notOperator == null) {
          // the is case
          _errorReporter.reportErrorForNode(HintCode.TYPE_CHECK_IS_NULL, node);
        } else {
          // the is not case
          _errorReporter.reportErrorForNode(
              HintCode.TYPE_CHECK_IS_NOT_NULL, node);
        }
        return true;
      }
    }
    return false;
  }

  void _checkForAbstractSuperMemberReference(
      Expression target, SimpleIdentifier name) {
    if (target is SuperExpression &&
        !_currentLibrary.context.analysisOptions.enableSuperMixins) {
      Element element = name.staticElement;
      if (element is ExecutableElement && element.isAbstract) {
        if (!_enclosingClass.hasNoSuchMethod) {
          ExecutableElement concrete = null;
          if (element.kind == ElementKind.METHOD) {
            concrete = _enclosingClass.lookUpInheritedConcreteMethod(
                element.displayName, _currentLibrary);
          } else if (element.kind == ElementKind.GETTER) {
            concrete = _enclosingClass.lookUpInheritedConcreteGetter(
                element.displayName, _currentLibrary);
          } else if (element.kind == ElementKind.SETTER) {
            concrete = _enclosingClass.lookUpInheritedConcreteSetter(
                element.displayName, _currentLibrary);
          }
          if (concrete == null) {
            _errorReporter.reportTypeErrorForNode(
                HintCode.ABSTRACT_SUPER_MEMBER_REFERENCE,
                name,
                [element.kind.displayName, name.name]);
          }
        }
      }
    }
  }

  /**
   * This verifies that the passed expression can be assigned to its corresponding parameters.
   *
   * This method corresponds to ErrorVerifier.checkForArgumentTypeNotAssignable.
   *
   * TODO (jwren) In the ErrorVerifier there are other warnings that we could have a corresponding
   * hint for: see other callers of ErrorVerifier.checkForArgumentTypeNotAssignable(..).
   *
   * @param expression the expression to evaluate
   * @param expectedStaticType the expected static type of the parameter
   * @param actualStaticType the actual static type of the argument
   * @param expectedPropagatedType the expected propagated type of the parameter, may be
   *          `null`
   * @param actualPropagatedType the expected propagated type of the parameter, may be `null`
   * @return `true` if and only if an hint code is generated on the passed node
   * See [HintCode.ARGUMENT_TYPE_NOT_ASSIGNABLE].
   */
  bool _checkForArgumentTypeNotAssignable(
      Expression expression,
      DartType expectedStaticType,
      DartType actualStaticType,
      DartType expectedPropagatedType,
      DartType actualPropagatedType,
      ErrorCode hintCode) {
    //
    // Warning case: test static type information
    //
    if (actualStaticType != null && expectedStaticType != null) {
      if (!_typeSystem.isAssignableTo(actualStaticType, expectedStaticType)) {
        // A warning was created in the ErrorVerifier, return false, don't
        // create a hint when a warning has already been created.
        return false;
      }
    }
    //
    // Hint case: test propagated type information
    //
    // Compute the best types to use.
    DartType expectedBestType = expectedPropagatedType ?? expectedStaticType;
    DartType actualBestType = actualPropagatedType ?? actualStaticType;
    if (actualBestType != null && expectedBestType != null) {
      if (!_typeSystem.isAssignableTo(actualBestType, expectedBestType)) {
        _errorReporter.reportTypeErrorForNode(
            hintCode, expression, [actualBestType, expectedBestType]);
        return true;
      }
    }
    return false;
  }

  /**
   * This verifies that the passed argument can be assigned to its corresponding parameter.
   *
   * This method corresponds to ErrorCode.checkForArgumentTypeNotAssignableForArgument.
   *
   * @param argument the argument to evaluate
   * @return `true` if and only if an hint code is generated on the passed node
   * See [HintCode.ARGUMENT_TYPE_NOT_ASSIGNABLE].
   */
  bool _checkForArgumentTypeNotAssignableForArgument(Expression argument) {
    if (argument == null) {
      return false;
    }
    ParameterElement staticParameterElement = argument.staticParameterElement;
    DartType staticParameterType = staticParameterElement?.type;
    ParameterElement propagatedParameterElement =
        argument.propagatedParameterElement;
    DartType propagatedParameterType = propagatedParameterElement?.type;
    return _checkForArgumentTypeNotAssignableWithExpectedTypes(
        argument,
        staticParameterType,
        propagatedParameterType,
        HintCode.ARGUMENT_TYPE_NOT_ASSIGNABLE);
  }

  /**
   * This verifies that the passed expression can be assigned to its corresponding parameters.
   *
   * This method corresponds to ErrorCode.checkForArgumentTypeNotAssignableWithExpectedTypes.
   *
   * @param expression the expression to evaluate
   * @param expectedStaticType the expected static type
   * @param expectedPropagatedType the expected propagated type, may be `null`
   * @return `true` if and only if an hint code is generated on the passed node
   * See [HintCode.ARGUMENT_TYPE_NOT_ASSIGNABLE].
   */
  bool _checkForArgumentTypeNotAssignableWithExpectedTypes(
          Expression expression,
          DartType expectedStaticType,
          DartType expectedPropagatedType,
          ErrorCode errorCode) =>
      _checkForArgumentTypeNotAssignable(
          expression,
          expectedStaticType,
          expression.staticType,
          expectedPropagatedType,
          expression.propagatedType,
          errorCode);

  /**
   * This verifies that the passed arguments can be assigned to their corresponding parameters.
   *
   * This method corresponds to ErrorCode.checkForArgumentTypesNotAssignableInList.
   *
   * @param node the arguments to evaluate
   * @return `true` if and only if an hint code is generated on the passed node
   * See [HintCode.ARGUMENT_TYPE_NOT_ASSIGNABLE].
   */
  bool _checkForArgumentTypesNotAssignableInList(ArgumentList argumentList) {
    if (argumentList == null) {
      return false;
    }
    bool problemReported = false;
    for (Expression argument in argumentList.arguments) {
      if (_checkForArgumentTypeNotAssignableForArgument(argument)) {
        problemReported = true;
      }
    }
    return problemReported;
  }

  /**
   * Produce a hint if the given [target] could have a value of `null`, and
   * [identifier] is not a name of a getter or a method that exists in the
   * class [Null].
   */
  void _checkForCanBeNullAfterNullAware(Expression target, Token operator,
      SimpleIdentifier propertyName, SimpleIdentifier methodName) {
    if (operator?.type == TokenType.QUESTION_PERIOD) {
      return;
    }
    bool isNullTypeMember() {
      if (propertyName != null) {
        String name = propertyName.name;
        return _nullType.lookUpGetter(name, _currentLibrary) != null;
      }
      if (methodName != null) {
        String name = methodName.name;
        return _nullType.lookUpMethod(name, _currentLibrary) != null;
      }
      return false;
    }

    target = target?.unParenthesized;
    if (target is MethodInvocation) {
      if (target.operator?.type == TokenType.QUESTION_PERIOD &&
          !isNullTypeMember()) {
        _errorReporter.reportErrorForNode(
            HintCode.CAN_BE_NULL_AFTER_NULL_AWARE, target);
      }
    } else if (target is PropertyAccess) {
      if (target.operator.type == TokenType.QUESTION_PERIOD &&
          !isNullTypeMember()) {
        _errorReporter.reportErrorForNode(
            HintCode.CAN_BE_NULL_AFTER_NULL_AWARE, target);
      }
    }
  }

  /**
   * Given some [Element], look at the associated metadata and report the use of the member if
   * it is declared as deprecated.
   *
   * @param element some element to check for deprecated use of
   * @param node the node use for the location of the error
   * @return `true` if and only if a hint code is generated on the passed node
   * See [HintCode.DEPRECATED_MEMBER_USE].
   */
  void _checkForDeprecatedMemberUse(Element element, AstNode node) {
    bool isDeprecated(Element element) {
      if (element == null) {
        return false;
      } else if (element is PropertyAccessorElement && element.isSynthetic) {
        // TODO(brianwilkerson) Why isn't this the implementation for PropertyAccessorElement?
        Element variable = element.variable;
        if (variable == null) {
          return false;
        }
        return variable.isDeprecated;
      }
      return element.isDeprecated;
    }

    if (!inDeprecatedMember && isDeprecated(element)) {
      String displayName = element.displayName;
      if (element is ConstructorElement) {
        // TODO(jwren) We should modify ConstructorElement.getDisplayName(),
        // or have the logic centralized elsewhere, instead of doing this logic
        // here.
        displayName = element.enclosingElement.displayName;
        if (!element.displayName.isEmpty) {
          displayName = "$displayName.${element.displayName}";
        }
      } else if (displayName == FunctionElement.CALL_METHOD_NAME &&
          node is MethodInvocation &&
          node.staticInvokeType is InterfaceType) {
        displayName = "${resolutionMap
            .staticInvokeTypeForInvocationExpression(node)
            .displayName}.${element.displayName}";
      }
      _errorReporter.reportErrorForNode(
          HintCode.DEPRECATED_MEMBER_USE, node, [displayName]);
    }
  }

  /**
   * For [SimpleIdentifier]s, only call [checkForDeprecatedMemberUse]
   * if the node is not in a declaration context.
   *
   * Also, if the identifier is a constructor name in a constructor invocation, then calls to the
   * deprecated constructor will be caught by
   * [visitInstanceCreationExpression] and
   * [visitSuperConstructorInvocation], and can be ignored by
   * this visit method.
   *
   * @param identifier some simple identifier to check for deprecated use of
   * @return `true` if and only if a hint code is generated on the passed node
   * See [HintCode.DEPRECATED_MEMBER_USE].
   */
  void _checkForDeprecatedMemberUseAtIdentifier(SimpleIdentifier identifier) {
    if (identifier.inDeclarationContext()) {
      return;
    }
    AstNode parent = identifier.parent;
    if ((parent is ConstructorName && identical(identifier, parent.name)) ||
        (parent is ConstructorDeclaration &&
            identical(identifier, parent.returnType)) ||
        (parent is SuperConstructorInvocation &&
            identical(identifier, parent.constructorName)) ||
        parent is HideCombinator) {
      return;
    }
    _checkForDeprecatedMemberUse(identifier.bestElement, identifier);
  }

  /**
   * Check for the passed binary expression for the [HintCode.DIVISION_OPTIMIZATION].
   *
   * @param node the binary expression to check
   * @return `true` if and only if a hint code is generated on the passed node
   * See [HintCode.DIVISION_OPTIMIZATION].
   */
  bool _checkForDivisionOptimizationHint(BinaryExpression node) {
    // Return if the operator is not '/'
    if (node.operator.type != TokenType.SLASH) {
      return false;
    }
    // Return if the '/' operator is not defined in core, or if we don't know
    // its static or propagated type
    MethodElement methodElement = node.bestElement;
    if (methodElement == null) {
      return false;
    }
    LibraryElement libraryElement = methodElement.library;
    if (libraryElement != null && !libraryElement.isDartCore) {
      return false;
    }
    // Report error if the (x/y) has toInt() invoked on it
    AstNode parent = node.parent;
    if (parent is ParenthesizedExpression) {
      ParenthesizedExpression parenthesizedExpression =
          _wrapParenthesizedExpression(parent);
      AstNode grandParent = parenthesizedExpression.parent;
      if (grandParent is MethodInvocation) {
        if (_TO_INT_METHOD_NAME == grandParent.methodName.name &&
            grandParent.argumentList.arguments.isEmpty) {
          _errorReporter.reportErrorForNode(
              HintCode.DIVISION_OPTIMIZATION, grandParent);
          return true;
        }
      }
    }
    return false;
  }

  void _checkForImmutable(ClassDeclaration node) {
    /**
     * Return `true` if the given class [element] is annotated with the
     * `@immutable` annotation.
     */
    bool isImmutable(ClassElement element) {
      for (ElementAnnotation annotation in element.metadata) {
        if (annotation.isImmutable) {
          return true;
        }
      }
      return false;
    }

    /**
     * Return `true` if the given class [element] or any superclass of it is
     * annotated with the `@immutable` annotation.
     */
    bool isOrInheritsImmutable(
        ClassElement element, HashSet<ClassElement> visited) {
      if (visited.add(element)) {
        if (isImmutable(element)) {
          return true;
        }
        for (InterfaceType interface in element.mixins) {
          if (isOrInheritsImmutable(interface.element, visited)) {
            return true;
          }
        }
        for (InterfaceType mixin in element.interfaces) {
          if (isOrInheritsImmutable(mixin.element, visited)) {
            return true;
          }
        }
        if (element.supertype != null) {
          return isOrInheritsImmutable(element.supertype.element, visited);
        }
      }
      return false;
    }

    /**
     * Return `true` if the given class [element] defines a non-final instance
     * field.
     */
    bool hasNonFinalInstanceField(ClassElement element) {
      for (FieldElement field in element.fields) {
        if (!field.isSynthetic && !field.isFinal && !field.isStatic) {
          return true;
        }
      }
      return false;
    }

    /**
     * Return `true` if the given class [element] defines or inherits a
     * non-final field.
     */
    bool hasOrInheritsNonFinalInstanceField(
        ClassElement element, HashSet<ClassElement> visited) {
      if (visited.add(element)) {
        if (hasNonFinalInstanceField(element)) {
          return true;
        }
        for (InterfaceType mixin in element.mixins) {
          if (hasNonFinalInstanceField(mixin.element)) {
            return true;
          }
        }
        if (element.supertype != null) {
          return hasOrInheritsNonFinalInstanceField(
              element.supertype.element, visited);
        }
      }
      return false;
    }

    ClassElement element = node.element;
    if (isOrInheritsImmutable(element, new HashSet<ClassElement>()) &&
        hasOrInheritsNonFinalInstanceField(
            element, new HashSet<ClassElement>())) {
      _errorReporter.reportErrorForNode(HintCode.MUST_BE_IMMUTABLE, node.name);
    }
  }

  /**
   * This verifies that the passed left hand side and right hand side represent a valid assignment.
   *
   * This method corresponds to ErrorVerifier.checkForInvalidAssignment.
   *
   * @param lhs the left hand side expression
   * @param rhs the right hand side expression
   * @return `true` if and only if an error code is generated on the passed node
   * See [HintCode.INVALID_ASSIGNMENT].
   */
  bool _checkForInvalidAssignment(Expression lhs, Expression rhs) {
    if (lhs == null || rhs == null) {
      return false;
    }
    VariableElement leftVariableElement = ErrorVerifier.getVariableElement(lhs);
    DartType leftType = (leftVariableElement == null)
        ? ErrorVerifier.getStaticType(lhs)
        : leftVariableElement.type;
    DartType staticRightType = ErrorVerifier.getStaticType(rhs);
    if (!_typeSystem.isAssignableTo(staticRightType, leftType)) {
      // The warning was generated on this rhs
      return false;
    }
    // Test for, and then generate the hint
    DartType bestRightType = rhs.bestType;
    if (leftType != null && bestRightType != null) {
      if (!_typeSystem.isAssignableTo(bestRightType, leftType)) {
        _errorReporter.reportTypeErrorForNode(
            HintCode.INVALID_ASSIGNMENT, rhs, [bestRightType, leftType]);
        return true;
      }
    }
    return false;
  }

  void _checkForInvalidFactory(MethodDeclaration decl) {
    // Check declaration.
    // Note that null return types are expected to be flagged by other analyses.
    DartType returnType = decl.returnType?.type;
    if (returnType is VoidType) {
      _errorReporter.reportErrorForNode(HintCode.INVALID_FACTORY_METHOD_DECL,
          decl.name, [decl.name.toString()]);
      return;
    }

    // Check implementation.

    FunctionBody body = decl.body;
    if (body is EmptyFunctionBody) {
      // Abstract methods are OK.
      return;
    }

    // `new Foo()` or `null`.
    bool factoryExpression(Expression expression) =>
        expression is InstanceCreationExpression || expression is NullLiteral;

    if (body is ExpressionFunctionBody && factoryExpression(body.expression)) {
      return;
    } else if (body is BlockFunctionBody) {
      NodeList<Statement> statements = body.block.statements;
      if (statements.isNotEmpty) {
        Statement last = statements.last;
        if (last is ReturnStatement && factoryExpression(last.expression)) {
          return;
        }
      }
    }

    _errorReporter.reportErrorForNode(HintCode.INVALID_FACTORY_METHOD_IMPL,
        decl.name, [decl.name.toString()]);
  }

  /**
   * Produces a hint if the given identifier is a protected closure, field or
   * getter/setter, method closure or invocation accessed outside a subclass.
   */
  void _checkForInvalidProtectedMemberAccess(SimpleIdentifier identifier) {
    if (identifier.inDeclarationContext()) {
      return;
    }

    bool isProtected(Element element) {
      if (element is PropertyAccessorElement &&
          element.enclosingElement is ClassElement &&
          (element.isProtected || element.variable.isProtected)) {
        return true;
      }
      if (element is MethodElement &&
          element.enclosingElement is ClassElement &&
          element.isProtected) {
        return true;
      }
      return false;
    }

    bool inCommentReference(SimpleIdentifier identifier) =>
        identifier.getAncestor((AstNode node) => node is CommentReference) !=
        null;

    bool inCurrentLibrary(Element element) =>
        element.library == _currentLibrary;

    Element element = identifier.bestElement;
    if (isProtected(element) &&
        !inCurrentLibrary(element) &&
        !inCommentReference(identifier)) {
      ClassElement definingClass = element.enclosingElement;
      ClassDeclaration accessingClass =
          identifier.getAncestor((AstNode node) => node is ClassDeclaration);
      if (accessingClass == null ||
          !_hasTypeOrSuperType(accessingClass.element, definingClass.type)) {
        _errorReporter.reportErrorForNode(
            HintCode.INVALID_USE_OF_PROTECTED_MEMBER,
            identifier,
            [identifier.name.toString(), definingClass.name]);
      }
    }
  }

  /**
   * Check that the imported library does not define a loadLibrary function. The import has already
   * been determined to be deferred when this is called.
   *
   * @param node the import directive to evaluate
   * @param importElement the [ImportElement] retrieved from the node
   * @return `true` if and only if an error code is generated on the passed node
   * See [CompileTimeErrorCode.IMPORT_DEFERRED_LIBRARY_WITH_LOAD_FUNCTION].
   */
  bool _checkForLoadLibraryFunction(
      ImportDirective node, ImportElement importElement) {
    LibraryElement importedLibrary = importElement.importedLibrary;
    if (importedLibrary == null) {
      return false;
    }
    if (importedLibrary.hasLoadLibraryFunction) {
      _errorReporter.reportErrorForNode(
          HintCode.IMPORT_DEFERRED_LIBRARY_WITH_LOAD_FUNCTION,
          node,
          [importedLibrary.name]);
      return true;
    }
    return false;
  }

  /**
   * Generate a hint for functions or methods that have a return type, but do not have a return
   * statement on all branches. At the end of blocks with no return, Dart implicitly returns
   * `null`, avoiding these implicit returns is considered a best practice.
   *
   * Note: for async functions/methods, this hint only applies when the
   * function has a return type that Future<Null> is not assignable to.
   *
   * @param node the binary expression to check
   * @param body the function body
   * @return `true` if and only if a hint code is generated on the passed node
   * See [HintCode.MISSING_RETURN].
   */
  void _checkForMissingReturn(TypeAnnotation returnType, FunctionBody body) {
    // Check that the method or function has a return type, and a function body
    if (returnType == null || body == null) {
      return;
    }
    // Check that the body is a BlockFunctionBody
    if (body is BlockFunctionBody) {
      // Generators are never required to have a return statement.
      if (body.isGenerator) {
        return;
      }
      // Check that the type is resolvable, and is not "void"
      DartType returnTypeType = returnType.type;
      if (returnTypeType == null || returnTypeType.isVoid) {
        return;
      }
      // For async, give no hint if the return type does not matter, i.e.
      // dynamic, Future<Null> or Future<dynamic>.
      if (body.isAsynchronous) {
        if (returnTypeType.isDynamic) {
          return;
        }
        if (returnTypeType is InterfaceType &&
            returnTypeType.isDartAsyncFuture) {
          DartType futureArgument = returnTypeType.typeArguments[0];
          if (futureArgument.isDynamic ||
              futureArgument.isDartCoreNull ||
              futureArgument.isObject) {
            return;
          }
        }
      }
      // Check the block for a return statement, if not, create the hint
      if (!ExitDetector.exits(body)) {
        _errorReporter.reportErrorForNode(
            HintCode.MISSING_RETURN, returnType, [returnTypeType.displayName]);
      }
    }
  }

  /**
   * Produce a hint if the given [condition] could have a value of `null`.
   */
  void _checkForPossibleNullCondition(Expression condition) {
    condition = condition?.unParenthesized;
    if (condition is BinaryExpression) {
      _checkForPossibleNullConditionInBinaryExpression(condition);
    } else if (condition is PrefixExpression) {
      _checkForPossibleNullConditionInPrefixExpression(condition);
    } else {
      _checkForPossibleNullConditionInSimpleExpression(condition);
    }
  }

  /**
   * Produce a hint if any of the parts of the given binary [condition] could
   * have a value of `null`.
   */
  void _checkForPossibleNullConditionInBinaryExpression(
      BinaryExpression condition) {
    TokenType type = condition.operator?.type;
    if (type == TokenType.AMPERSAND_AMPERSAND || type == TokenType.BAR_BAR) {
      _checkForPossibleNullCondition(condition.leftOperand);
      _checkForPossibleNullCondition(condition.rightOperand);
    }
  }

  /**
   * Produce a hint if the operand of the given prefix [condition] could
   * have a value of `null`.
   */
  void _checkForPossibleNullConditionInPrefixExpression(
      PrefixExpression condition) {
    if (condition.operator?.type == TokenType.BANG) {
      _checkForPossibleNullCondition(condition.operand);
    }
  }

  /**
   * Produce a hint if the given [condition] could have a value of `null`.
   */
  void _checkForPossibleNullConditionInSimpleExpression(Expression condition) {
    if (condition is MethodInvocation) {
      if (condition.operator?.type == TokenType.QUESTION_PERIOD) {
        _errorReporter.reportErrorForNode(
            HintCode.NULL_AWARE_IN_CONDITION, condition);
      }
    } else if (condition is PropertyAccess) {
      if (condition.operator?.type == TokenType.QUESTION_PERIOD) {
        _errorReporter.reportErrorForNode(
            HintCode.NULL_AWARE_IN_CONDITION, condition);
      }
    }
  }

  /**
   * Check for the passed as expression for the [HintCode.UNNECESSARY_CAST] hint code.
   *
   * @param node the as expression to check
   * @return `true` if and only if a hint code is generated on the passed node
   * See [HintCode.UNNECESSARY_CAST].
   */
  bool _checkForUnnecessaryCast(AsExpression node) {
    // TODO(jwren) After dartbug.com/13732, revisit this, we should be able to
    // remove the (x is! TypeParameterType) checks.
    AstNode parent = node.parent;
    if (parent is ConditionalExpression &&
        (node == parent.thenExpression || node == parent.elseExpression)) {
      Expression thenExpression = parent.thenExpression;
      DartType thenType;
      if (thenExpression is AsExpression) {
        thenType = thenExpression.expression.staticType;
      } else {
        thenType = thenExpression.staticType;
      }
      Expression elseExpression = parent.elseExpression;
      DartType elseType;
      if (elseExpression is AsExpression) {
        elseType = elseExpression.expression.staticType;
      } else {
        elseType = elseExpression.staticType;
      }
      if (thenType != null &&
          elseType != null &&
          !thenType.isDynamic &&
          !elseType.isDynamic &&
          !thenType.isMoreSpecificThan(elseType) &&
          !elseType.isMoreSpecificThan(thenType)) {
        return false;
      }
    }
    DartType lhsType = node.expression.staticType;
    DartType rhsType = node.type.type;
    if (lhsType != null &&
        rhsType != null &&
        !lhsType.isDynamic &&
        !rhsType.isDynamic &&
        lhsType.isMoreSpecificThan(rhsType)) {
      _errorReporter.reportErrorForNode(HintCode.UNNECESSARY_CAST, node);
      return true;
    }
    return false;
  }

  /**
   * Generate a hint for `noSuchMethod` methods that do nothing except of
   * calling another `noSuchMethod` that is not defined by `Object`.
   *
   * @return `true` if and only if a hint code is generated on the passed node
   * See [HintCode.UNNECESSARY_NO_SUCH_METHOD].
   */
  bool _checkForUnnecessaryNoSuchMethod(MethodDeclaration node) {
    if (node.name.name != FunctionElement.NO_SUCH_METHOD_METHOD_NAME) {
      return false;
    }
    bool isNonObjectNoSuchMethodInvocation(Expression invocation) {
      if (invocation is MethodInvocation &&
          invocation.target is SuperExpression &&
          invocation.argumentList.arguments.length == 1) {
        SimpleIdentifier name = invocation.methodName;
        if (name.name == FunctionElement.NO_SUCH_METHOD_METHOD_NAME) {
          Element methodElement = name.staticElement;
          Element classElement = methodElement?.enclosingElement;
          return methodElement is MethodElement &&
              classElement is ClassElement &&
              !classElement.type.isObject;
        }
      }
      return false;
    }

    FunctionBody body = node.body;
    if (body is ExpressionFunctionBody) {
      if (isNonObjectNoSuchMethodInvocation(body.expression)) {
        _errorReporter.reportErrorForNode(
            HintCode.UNNECESSARY_NO_SUCH_METHOD, node);
        return true;
      }
    } else if (body is BlockFunctionBody) {
      List<Statement> statements = body.block.statements;
      if (statements.length == 1) {
        Statement returnStatement = statements.first;
        if (returnStatement is ReturnStatement &&
            isNonObjectNoSuchMethodInvocation(returnStatement.expression)) {
          _errorReporter.reportErrorForNode(
              HintCode.UNNECESSARY_NO_SUCH_METHOD, node);
          return true;
        }
      }
    }
    return false;
  }

  /**
   * Check for situations where the result of a method or function is used, when
   * it returns 'void'.
   *
   * See [HintCode.USE_OF_VOID_RESULT].
   */
  void _checkForUseOfVoidResult(Expression expression) {
    // TODO(jwren) Many other situations of use could be covered. We currently
    // cover the cases var x = m() and x = m(), but we could also cover cases
    // such as m().x, m()[k], a + m(), f(m()), return m().
    if (expression is MethodInvocation) {
      if (identical(expression.staticType, VoidTypeImpl.instance)) {
        SimpleIdentifier methodName = expression.methodName;
        _errorReporter.reportErrorForNode(
            HintCode.USE_OF_VOID_RESULT, methodName, [methodName.name]);
      }
    }
  }

  /**
   * Check for the passed class declaration for the
   * [HintCode.OVERRIDE_EQUALS_BUT_NOT_HASH_CODE] hint code.
   *
   * @param node the class declaration to check
   * @return `true` if and only if a hint code is generated on the passed node
   * See [HintCode.OVERRIDE_EQUALS_BUT_NOT_HASH_CODE].
   */
//  bool _checkForOverrideEqualsButNotHashCode(ClassDeclaration node) {
//    ClassElement classElement = node.element;
//    if (classElement == null) {
//      return false;
//    }
//    MethodElement equalsOperatorMethodElement =
//        classElement.getMethod(sc.TokenType.EQ_EQ.lexeme);
//    if (equalsOperatorMethodElement != null) {
//      PropertyAccessorElement hashCodeElement =
//          classElement.getGetter(_HASHCODE_GETTER_NAME);
//      if (hashCodeElement == null) {
//        _errorReporter.reportErrorForNode(
//            HintCode.OVERRIDE_EQUALS_BUT_NOT_HASH_CODE,
//            node.name,
//            [classElement.displayName]);
//        return true;
//      }
//    }
//    return false;
//  }

  bool _hasTypeOrSuperType(ClassElement element, InterfaceType type) {
    if (element == null) {
      return false;
    }
    ClassElement typeElement = type.element;
    return element == typeElement ||
        element.allSupertypes
            .any((InterfaceType t) => t.element == typeElement);
  }

  /**
   * Given a parenthesized expression, this returns the parent (or recursively grand-parent) of the
   * expression that is a parenthesized expression, but whose parent is not a parenthesized
   * expression.
   *
   * For example given the code `(((e)))`: `(e) -> (((e)))`.
   *
   * @param parenthesizedExpression some expression whose parent is a parenthesized expression
   * @return the first parent or grand-parent that is a parenthesized expression, that does not have
   *         a parenthesized expression parent
   */
  static ParenthesizedExpression _wrapParenthesizedExpression(
      ParenthesizedExpression parenthesizedExpression) {
    AstNode parent = parenthesizedExpression.parent;
    if (parent is ParenthesizedExpression) {
      return _wrapParenthesizedExpression(parent);
    }
    return parenthesizedExpression;
  }
}

/**
 * Utilities for [LibraryElementImpl] building.
 */
class BuildLibraryElementUtils {
  /**
   * Look through all of the compilation units defined for the given [library],
   * looking for getters and setters that are defined in different compilation
   * units but that have the same names. If any are found, make sure that they
   * have the same variable element.
   */
  static void patchTopLevelAccessors(LibraryElementImpl library) {
    // Without parts getters/setters already share the same variable element.
    List<CompilationUnitElement> parts = library.parts;
    if (parts.isEmpty) {
      return;
    }
    // Collect getters and setters.
    HashMap<String, PropertyAccessorElement> getters =
        new HashMap<String, PropertyAccessorElement>();
    List<PropertyAccessorElement> setters = <PropertyAccessorElement>[];
    _collectAccessors(getters, setters, library.definingCompilationUnit);
    int partLength = parts.length;
    for (int i = 0; i < partLength; i++) {
      CompilationUnitElement unit = parts[i];
      _collectAccessors(getters, setters, unit);
    }
    // Move every setter to the corresponding getter's variable (if exists).
    int setterLength = setters.length;
    for (int j = 0; j < setterLength; j++) {
      PropertyAccessorElement setter = setters[j];
      PropertyAccessorElement getter = getters[setter.displayName];
      if (getter != null) {
        TopLevelVariableElementImpl variable = getter.variable;
        TopLevelVariableElementImpl setterVariable = setter.variable;
        CompilationUnitElementImpl setterUnit = setterVariable.enclosingElement;
        setterUnit.replaceTopLevelVariable(setterVariable, variable);
        variable.setter = setter;
        (setter as PropertyAccessorElementImpl).variable = variable;
      }
    }
  }

  /**
   * Add all of the non-synthetic [getters] and [setters] defined in the given
   * [unit] that have no corresponding accessor to one of the given collections.
   */
  static void _collectAccessors(Map<String, PropertyAccessorElement> getters,
      List<PropertyAccessorElement> setters, CompilationUnitElement unit) {
    List<PropertyAccessorElement> accessors = unit.accessors;
    int length = accessors.length;
    for (int i = 0; i < length; i++) {
      PropertyAccessorElement accessor = accessors[i];
      if (accessor.isGetter) {
        if (!accessor.isSynthetic && accessor.correspondingSetter == null) {
          getters[accessor.displayName] = accessor;
        }
      } else {
        if (!accessor.isSynthetic && accessor.correspondingGetter == null) {
          setters.add(accessor);
        }
      }
    }
  }
}

/**
 * Instances of the class `ConstantVerifier` traverse an AST structure looking for additional
 * errors and warnings not covered by the parser and resolver. In particular, it looks for errors
 * and warnings related to constant expressions.
 */
class ConstantVerifier extends RecursiveAstVisitor<Object> {
  /**
   * The error reporter by which errors will be reported.
   */
  final ErrorReporter _errorReporter;

  /**
   * The type provider used to access the known types.
   */
  final TypeProvider _typeProvider;

  /**
   * The type system in use.
   */
  final TypeSystem _typeSystem;

  /**
   * The set of variables declared using '-D' on the command line.
   */
  final DeclaredVariables declaredVariables;

  /**
   * The type representing the type 'bool'.
   */
  InterfaceType _boolType;

  /**
   * The type representing the type 'int'.
   */
  InterfaceType _intType;

  /**
   * The type representing the type 'num'.
   */
  InterfaceType _numType;

  /**
   * The type representing the type 'string'.
   */
  InterfaceType _stringType;

  /**
   * The current library that is being analyzed.
   */
  final LibraryElement _currentLibrary;

  /**
   * Initialize a newly created constant verifier.
   *
   * @param errorReporter the error reporter by which errors will be reported
   */
  ConstantVerifier(this._errorReporter, LibraryElement currentLibrary,
      this._typeProvider, this.declaredVariables)
      : _currentLibrary = currentLibrary,
        _typeSystem = currentLibrary.context.typeSystem {
    this._boolType = _typeProvider.boolType;
    this._intType = _typeProvider.intType;
    this._numType = _typeProvider.numType;
    this._stringType = _typeProvider.stringType;
  }

  @override
  Object visitAnnotation(Annotation node) {
    super.visitAnnotation(node);
    // check annotation creation
    Element element = node.element;
    if (element is ConstructorElement) {
      // should be 'const' constructor
      if (!element.isConst) {
        _errorReporter.reportErrorForNode(
            CompileTimeErrorCode.NON_CONSTANT_ANNOTATION_CONSTRUCTOR, node);
        return null;
      }
      // should have arguments
      ArgumentList argumentList = node.arguments;
      if (argumentList == null) {
        _errorReporter.reportErrorForNode(
            CompileTimeErrorCode.NO_ANNOTATION_CONSTRUCTOR_ARGUMENTS, node);
        return null;
      }
      // arguments should be constants
      _validateConstantArguments(argumentList);
    }
    return null;
  }

  @override
  Object visitConstructorDeclaration(ConstructorDeclaration node) {
    if (node.constKeyword != null) {
      _validateConstructorInitializers(node);
      _validateFieldInitializers(node.parent as ClassDeclaration, node);
    }
    _validateDefaultValues(node.parameters);
    return super.visitConstructorDeclaration(node);
  }

  @override
  Object visitFunctionExpression(FunctionExpression node) {
    super.visitFunctionExpression(node);
    _validateDefaultValues(node.parameters);
    return null;
  }

  @override
  Object visitInstanceCreationExpression(InstanceCreationExpression node) {
    if (node.isConst) {
      // We need to evaluate the constant to see if any errors occur during its
      // evaluation.
      ConstructorElement constructor = node.staticElement;
      if (constructor != null) {
        ConstantEvaluationEngine evaluationEngine =
            new ConstantEvaluationEngine(_typeProvider, declaredVariables,
                typeSystem: _typeSystem);
        ConstantVisitor constantVisitor =
            new ConstantVisitor(evaluationEngine, _errorReporter);
        evaluationEngine.evaluateConstructorCall(
            node,
            node.argumentList.arguments,
            constructor,
            constantVisitor,
            _errorReporter);
      }
    }
    _validateInstanceCreationArguments(node);
    return super.visitInstanceCreationExpression(node);
  }

  @override
  Object visitListLiteral(ListLiteral node) {
    super.visitListLiteral(node);
    if (node.constKeyword != null) {
      DartObjectImpl result;
      for (Expression element in node.elements) {
        result =
            _validate(element, CompileTimeErrorCode.NON_CONSTANT_LIST_ELEMENT);
        if (result != null) {
          _reportErrorIfFromDeferredLibrary(
              element,
              CompileTimeErrorCode
                  .NON_CONSTANT_LIST_ELEMENT_FROM_DEFERRED_LIBRARY);
        }
      }
    }
    return null;
  }

  @override
  Object visitMapLiteral(MapLiteral node) {
    super.visitMapLiteral(node);
    bool isConst = node.constKeyword != null;
    bool reportEqualKeys = true;
    HashSet<DartObject> keys = new HashSet<DartObject>();
    List<Expression> invalidKeys = new List<Expression>();
    for (MapLiteralEntry entry in node.entries) {
      Expression key = entry.key;
      if (isConst) {
        DartObjectImpl keyResult =
            _validate(key, CompileTimeErrorCode.NON_CONSTANT_MAP_KEY);
        Expression valueExpression = entry.value;
        DartObjectImpl valueResult = _validate(
            valueExpression, CompileTimeErrorCode.NON_CONSTANT_MAP_VALUE);
        if (valueResult != null) {
          _reportErrorIfFromDeferredLibrary(
              valueExpression,
              CompileTimeErrorCode
                  .NON_CONSTANT_MAP_VALUE_FROM_DEFERRED_LIBRARY);
        }
        if (keyResult != null) {
          _reportErrorIfFromDeferredLibrary(key,
              CompileTimeErrorCode.NON_CONSTANT_MAP_KEY_FROM_DEFERRED_LIBRARY);
          if (keys.contains(keyResult)) {
            invalidKeys.add(key);
          } else {
            keys.add(keyResult);
          }
          DartType type = keyResult.type;
          if (_implementsEqualsWhenNotAllowed(type)) {
            _errorReporter.reportErrorForNode(
                CompileTimeErrorCode
                    .CONST_MAP_KEY_EXPRESSION_TYPE_IMPLEMENTS_EQUALS,
                key,
                [type.displayName]);
          }
        }
      } else {
        // Note: we throw the errors away because this isn't actually a const.
        AnalysisErrorListener errorListener =
            AnalysisErrorListener.NULL_LISTENER;
        ErrorReporter subErrorReporter =
            new ErrorReporter(errorListener, _errorReporter.source);
        DartObjectImpl result = key.accept(new ConstantVisitor(
            new ConstantEvaluationEngine(_typeProvider, declaredVariables,
                typeSystem: _typeSystem),
            subErrorReporter));
        if (result != null) {
          if (keys.contains(result)) {
            invalidKeys.add(key);
          } else {
            keys.add(result);
          }
        } else {
          reportEqualKeys = false;
        }
      }
    }
    if (reportEqualKeys) {
      int length = invalidKeys.length;
      for (int i = 0; i < length; i++) {
        _errorReporter.reportErrorForNode(
            StaticWarningCode.EQUAL_KEYS_IN_MAP, invalidKeys[i]);
      }
    }
    return null;
  }

  @override
  Object visitMethodDeclaration(MethodDeclaration node) {
    super.visitMethodDeclaration(node);
    _validateDefaultValues(node.parameters);
    return null;
  }

  @override
  Object visitSwitchStatement(SwitchStatement node) {
    // TODO(paulberry): to minimize error messages, it would be nice to
    // compare all types with the most popular type rather than the first
    // type.
    NodeList<SwitchMember> switchMembers = node.members;
    bool foundError = false;
    DartType firstType = null;
    for (SwitchMember switchMember in switchMembers) {
      if (switchMember is SwitchCase) {
        Expression expression = switchMember.expression;
        DartObjectImpl caseResult = _validate(
            expression, CompileTimeErrorCode.NON_CONSTANT_CASE_EXPRESSION);
        if (caseResult != null) {
          _reportErrorIfFromDeferredLibrary(
              expression,
              CompileTimeErrorCode
                  .NON_CONSTANT_CASE_EXPRESSION_FROM_DEFERRED_LIBRARY);
          DartObject value = caseResult;
          if (firstType == null) {
            firstType = value.type;
          } else {
            DartType nType = value.type;
            if (firstType != nType) {
              _errorReporter.reportErrorForNode(
                  CompileTimeErrorCode.INCONSISTENT_CASE_EXPRESSION_TYPES,
                  expression,
                  [expression.toSource(), firstType.displayName]);
              foundError = true;
            }
          }
        }
      }
    }
    if (!foundError) {
      _checkForCaseExpressionTypeImplementsEquals(node, firstType);
    }
    return super.visitSwitchStatement(node);
  }

  @override
  Object visitVariableDeclaration(VariableDeclaration node) {
    super.visitVariableDeclaration(node);
    Expression initializer = node.initializer;
    if (initializer != null && (node.isConst || node.isFinal)) {
      VariableElementImpl element = node.element as VariableElementImpl;
      EvaluationResultImpl result = element.evaluationResult;
      if (result == null) {
        // Variables marked "const" should have had their values computed by
        // ConstantValueComputer.  Other variables will only have had their
        // values computed if the value was needed (e.g. final variables in a
        // class containing const constructors).
        assert(!node.isConst);
        return null;
      }
      _reportErrors(result.errors,
          CompileTimeErrorCode.CONST_INITIALIZED_WITH_NON_CONSTANT_VALUE);
      _reportErrorIfFromDeferredLibrary(
          initializer,
          CompileTimeErrorCode
              .CONST_INITIALIZED_WITH_NON_CONSTANT_VALUE_FROM_DEFERRED_LIBRARY);
    }
    return null;
  }

  /**
   * This verifies that the passed switch statement does not have a case expression with the
   * operator '==' overridden.
   *
   * @param node the switch statement to evaluate
   * @param type the common type of all 'case' expressions
   * @return `true` if and only if an error code is generated on the passed node
   * See [CompileTimeErrorCode.CASE_EXPRESSION_TYPE_IMPLEMENTS_EQUALS].
   */
  bool _checkForCaseExpressionTypeImplementsEquals(
      SwitchStatement node, DartType type) {
    if (!_implementsEqualsWhenNotAllowed(type)) {
      return false;
    }
    // report error
    _errorReporter.reportErrorForToken(
        CompileTimeErrorCode.CASE_EXPRESSION_TYPE_IMPLEMENTS_EQUALS,
        node.switchKeyword,
        [type.displayName]);
    return true;
  }

  /**
   * @return `true` if given [Type] implements operator <i>==</i>, and it is not
   *         <i>int</i> or <i>String</i>.
   */
  bool _implementsEqualsWhenNotAllowed(DartType type) {
    // ignore int or String
    if (type == null || type == _intType || type == _typeProvider.stringType) {
      return false;
    } else if (type == _typeProvider.doubleType) {
      return true;
    }
    // prepare ClassElement
    Element element = type.element;
    if (element is ClassElement) {
      // lookup for ==
      MethodElement method =
          element.lookUpConcreteMethod("==", _currentLibrary);
      if (method == null || method.enclosingElement.type.isObject) {
        return false;
      }
      // there is == that we don't like
      return true;
    }
    return false;
  }

  /**
   * Given some computed [Expression], this method generates the passed [ErrorCode] on
   * the node if its' value consists of information from a deferred library.
   *
   * @param expression the expression to be tested for a deferred library reference
   * @param errorCode the error code to be used if the expression is or consists of a reference to a
   *          deferred library
   */
  void _reportErrorIfFromDeferredLibrary(
      Expression expression, ErrorCode errorCode) {
    DeferredLibraryReferenceDetector referenceDetector =
        new DeferredLibraryReferenceDetector();
    expression.accept(referenceDetector);
    if (referenceDetector.result) {
      _errorReporter.reportErrorForNode(errorCode, expression);
    }
  }

  /**
   * Report any errors in the given list. Except for special cases, use the given error code rather
   * than the one reported in the error.
   *
   * @param errors the errors that need to be reported
   * @param errorCode the error code to be used
   */
  void _reportErrors(List<AnalysisError> errors, ErrorCode errorCode) {
    int length = errors.length;
    for (int i = 0; i < length; i++) {
      AnalysisError data = errors[i];
      ErrorCode dataErrorCode = data.errorCode;
      if (identical(dataErrorCode,
              CompileTimeErrorCode.CONST_EVAL_THROWS_EXCEPTION) ||
          identical(
              dataErrorCode, CompileTimeErrorCode.CONST_EVAL_THROWS_IDBZE) ||
          identical(dataErrorCode,
              CompileTimeErrorCode.CONST_EVAL_TYPE_BOOL_NUM_STRING) ||
          identical(dataErrorCode, CompileTimeErrorCode.CONST_EVAL_TYPE_BOOL) ||
          identical(dataErrorCode, CompileTimeErrorCode.CONST_EVAL_TYPE_INT) ||
          identical(dataErrorCode, CompileTimeErrorCode.CONST_EVAL_TYPE_NUM) ||
          identical(dataErrorCode,
              CompileTimeErrorCode.RECURSIVE_COMPILE_TIME_CONSTANT) ||
          identical(
              dataErrorCode,
              CheckedModeCompileTimeErrorCode
                  .CONST_CONSTRUCTOR_FIELD_TYPE_MISMATCH) ||
          identical(
              dataErrorCode,
              CheckedModeCompileTimeErrorCode
                  .CONST_CONSTRUCTOR_PARAM_TYPE_MISMATCH) ||
          identical(dataErrorCode,
              CheckedModeCompileTimeErrorCode.VARIABLE_TYPE_MISMATCH)) {
        _errorReporter.reportError(data);
      } else if (errorCode != null) {
        _errorReporter.reportError(new AnalysisError(
            data.source, data.offset, data.length, errorCode));
      }
    }
  }

  /**
   * Validate that the given expression is a compile time constant. Return the value of the compile
   * time constant, or `null` if the expression is not a compile time constant.
   *
   * @param expression the expression to be validated
   * @param errorCode the error code to be used if the expression is not a compile time constant
   * @return the value of the compile time constant
   */
  DartObjectImpl _validate(Expression expression, ErrorCode errorCode) {
    RecordingErrorListener errorListener = new RecordingErrorListener();
    ErrorReporter subErrorReporter =
        new ErrorReporter(errorListener, _errorReporter.source);
    DartObjectImpl result = expression.accept(new ConstantVisitor(
        new ConstantEvaluationEngine(_typeProvider, declaredVariables,
            typeSystem: _typeSystem),
        subErrorReporter));
    _reportErrors(errorListener.errors, errorCode);
    return result;
  }

  /**
   * Validate that if the passed arguments are constant expressions.
   *
   * @param argumentList the argument list to evaluate
   */
  void _validateConstantArguments(ArgumentList argumentList) {
    for (Expression argument in argumentList.arguments) {
      Expression realArgument =
          argument is NamedExpression ? argument.expression : argument;
      _validate(
          realArgument, CompileTimeErrorCode.CONST_WITH_NON_CONSTANT_ARGUMENT);
    }
  }

  /**
   * Validates that the expressions of the initializers of the given constant
   * [constructor] are all compile time constants.
   */
  void _validateConstructorInitializers(ConstructorDeclaration constructor) {
    List<ParameterElement> parameterElements =
        constructor.parameters.parameterElements;
    NodeList<ConstructorInitializer> initializers = constructor.initializers;
    for (ConstructorInitializer initializer in initializers) {
      if (initializer is AssertInitializer) {
        _validateInitializerExpression(
            parameterElements, initializer.condition);
        Expression message = initializer.message;
        if (message != null) {
          _validateInitializerExpression(parameterElements, message);
        }
      } else if (initializer is ConstructorFieldInitializer) {
        _validateInitializerExpression(
            parameterElements, initializer.expression);
      } else if (initializer is RedirectingConstructorInvocation) {
        _validateInitializerInvocationArguments(
            parameterElements, initializer.argumentList);
      } else if (initializer is SuperConstructorInvocation) {
        _validateInitializerInvocationArguments(
            parameterElements, initializer.argumentList);
      }
    }
  }

  /**
   * Validate that the default value associated with each of the parameters in the given list is a
   * compile time constant.
   *
   * @param parameters the list of parameters to be validated
   */
  void _validateDefaultValues(FormalParameterList parameters) {
    if (parameters == null) {
      return;
    }
    for (FormalParameter parameter in parameters.parameters) {
      if (parameter is DefaultFormalParameter) {
        Expression defaultValue = parameter.defaultValue;
        DartObjectImpl result;
        if (defaultValue == null) {
          result =
              new DartObjectImpl(_typeProvider.nullType, NullState.NULL_STATE);
        } else {
          result = _validate(
              defaultValue, CompileTimeErrorCode.NON_CONSTANT_DEFAULT_VALUE);
          if (result != null) {
            _reportErrorIfFromDeferredLibrary(
                defaultValue,
                CompileTimeErrorCode
                    .NON_CONSTANT_DEFAULT_VALUE_FROM_DEFERRED_LIBRARY);
          }
        }
        VariableElementImpl element = parameter.element as VariableElementImpl;
        element.evaluationResult = new EvaluationResultImpl(result);
      }
    }
  }

  /**
   * Validates that the expressions of any field initializers in the class declaration are all
   * compile time constants. Since this is only required if the class has a constant constructor,
   * the error is reported at the constructor site.
   *
   * @param classDeclaration the class which should be validated
   * @param errorSite the site at which errors should be reported.
   */
  void _validateFieldInitializers(
      ClassDeclaration classDeclaration, ConstructorDeclaration errorSite) {
    NodeList<ClassMember> members = classDeclaration.members;
    for (ClassMember member in members) {
      if (member is FieldDeclaration && !member.isStatic) {
        for (VariableDeclaration variableDeclaration
            in member.fields.variables) {
          Expression initializer = variableDeclaration.initializer;
          if (initializer != null) {
            // Ignore any errors produced during validation--if the constant
            // can't be eavluated we'll just report a single error.
            AnalysisErrorListener errorListener =
                AnalysisErrorListener.NULL_LISTENER;
            ErrorReporter subErrorReporter =
                new ErrorReporter(errorListener, _errorReporter.source);
            DartObjectImpl result = initializer.accept(new ConstantVisitor(
                new ConstantEvaluationEngine(_typeProvider, declaredVariables,
                    typeSystem: _typeSystem),
                subErrorReporter));
            if (result == null) {
              _errorReporter.reportErrorForNode(
                  CompileTimeErrorCode
                      .CONST_CONSTRUCTOR_WITH_FIELD_INITIALIZED_BY_NON_CONST,
                  errorSite,
                  [variableDeclaration.name.name]);
            }
          }
        }
      }
    }
  }

  /**
   * Validates that the given expression is a compile time constant.
   *
   * @param parameterElements the elements of parameters of constant constructor, they are
   *          considered as a valid potentially constant expressions
   * @param expression the expression to validate
   */
  void _validateInitializerExpression(
      List<ParameterElement> parameterElements, Expression expression) {
    RecordingErrorListener errorListener = new RecordingErrorListener();
    ErrorReporter subErrorReporter =
        new ErrorReporter(errorListener, _errorReporter.source);
    DartObjectImpl result = expression.accept(
        new _ConstantVerifier_validateInitializerExpression(_typeProvider,
            subErrorReporter, this, parameterElements, declaredVariables,
            typeSystem: _typeSystem));
    _reportErrors(errorListener.errors,
        CompileTimeErrorCode.NON_CONSTANT_VALUE_IN_INITIALIZER);
    if (result != null) {
      _reportErrorIfFromDeferredLibrary(
          expression,
          CompileTimeErrorCode
              .NON_CONSTANT_VALUE_IN_INITIALIZER_FROM_DEFERRED_LIBRARY);
    }
  }

  /**
   * Validates that all of the arguments of a constructor initializer are compile time constants.
   *
   * @param parameterElements the elements of parameters of constant constructor, they are
   *          considered as a valid potentially constant expressions
   * @param argumentList the argument list to validate
   */
  void _validateInitializerInvocationArguments(
      List<ParameterElement> parameterElements, ArgumentList argumentList) {
    if (argumentList == null) {
      return;
    }
    for (Expression argument in argumentList.arguments) {
      _validateInitializerExpression(parameterElements, argument);
    }
  }

  /**
   * Validate that if the passed instance creation is 'const' then all its arguments are constant
   * expressions.
   *
   * @param node the instance creation evaluate
   */
  void _validateInstanceCreationArguments(InstanceCreationExpression node) {
    if (!node.isConst) {
      return;
    }
    ArgumentList argumentList = node.argumentList;
    if (argumentList == null) {
      return;
    }
    _validateConstantArguments(argumentList);
  }
}

/**
 * Instances of the class `Dart2JSVerifier` traverse an AST structure looking for hints for
 * code that will be compiled to JS, such as [HintCode.IS_DOUBLE].
 */
class Dart2JSVerifier extends RecursiveAstVisitor<Object> {
  /**
   * The name of the `double` type.
   */
  static String _DOUBLE_TYPE_NAME = "double";

  /**
   * The error reporter by which errors will be reported.
   */
  final ErrorReporter _errorReporter;

  /**
   * Create a new instance of the [Dart2JSVerifier].
   *
   * @param errorReporter the error reporter
   */
  Dart2JSVerifier(this._errorReporter);

  @override
  Object visitIsExpression(IsExpression node) {
    _checkForIsDoubleHints(node);
    return super.visitIsExpression(node);
  }

  /**
   * Check for instances of `x is double`, `x is int`, `x is! double` and
   * `x is! int`.
   *
   * @param node the is expression to check
   * @return `true` if and only if a hint code is generated on the passed node
   * See [HintCode.IS_DOUBLE],
   * [HintCode.IS_INT],
   * [HintCode.IS_NOT_DOUBLE], and
   * [HintCode.IS_NOT_INT].
   */
  bool _checkForIsDoubleHints(IsExpression node) {
    DartType type = node.type.type;
    Element element = type?.element;
    if (element != null) {
      String typeNameStr = element.name;
      LibraryElement libraryElement = element.library;
      //      if (typeNameStr.equals(INT_TYPE_NAME) && libraryElement != null
      //          && libraryElement.isDartCore()) {
      //        if (node.getNotOperator() == null) {
      //          errorReporter.reportError(HintCode.IS_INT, node);
      //        } else {
      //          errorReporter.reportError(HintCode.IS_NOT_INT, node);
      //        }
      //        return true;
      //      } else
      if (typeNameStr == _DOUBLE_TYPE_NAME &&
          libraryElement != null &&
          libraryElement.isDartCore) {
        if (node.notOperator == null) {
          _errorReporter.reportErrorForNode(HintCode.IS_DOUBLE, node);
        } else {
          _errorReporter.reportErrorForNode(HintCode.IS_NOT_DOUBLE, node);
        }
        return true;
      }
    }
    return false;
  }
}

/**
 * Instances of the class `DeadCodeVerifier` traverse an AST structure looking for cases of
 * [HintCode.DEAD_CODE].
 */
class DeadCodeVerifier extends RecursiveAstVisitor<Object> {
  /**
   * The error reporter by which errors will be reported.
   */
  final ErrorReporter _errorReporter;

  /**
   *  The type system for this visitor
   */
  final TypeSystem _typeSystem;

  /**
   * Create a new instance of the [DeadCodeVerifier].
   *
   * @param errorReporter the error reporter
   */
  DeadCodeVerifier(this._errorReporter, {TypeSystem typeSystem})
      : this._typeSystem = typeSystem ?? new TypeSystemImpl(null);

  @override
  Object visitBinaryExpression(BinaryExpression node) {
    Token operator = node.operator;
    bool isAmpAmp = operator.type == TokenType.AMPERSAND_AMPERSAND;
    bool isBarBar = operator.type == TokenType.BAR_BAR;
    if (isAmpAmp || isBarBar) {
      Expression lhsCondition = node.leftOperand;
      if (!_isDebugConstant(lhsCondition)) {
        EvaluationResultImpl lhsResult = _getConstantBooleanValue(lhsCondition);
        if (lhsResult != null) {
          bool value = lhsResult.value.toBoolValue();
          if (value == true && isBarBar) {
            // report error on else block: true || !e!
            _errorReporter.reportErrorForNode(
                HintCode.DEAD_CODE, node.rightOperand);
            // only visit the LHS:
            lhsCondition?.accept(this);
            return null;
          } else if (value == false && isAmpAmp) {
            // report error on if block: false && !e!
            _errorReporter.reportErrorForNode(
                HintCode.DEAD_CODE, node.rightOperand);
            // only visit the LHS:
            lhsCondition?.accept(this);
            return null;
          }
        }
      }
      // How do we want to handle the RHS? It isn't dead code, but "pointless"
      // or "obscure"...
//            Expression rhsCondition = node.getRightOperand();
//            ValidResult rhsResult = getConstantBooleanValue(rhsCondition);
//            if (rhsResult != null) {
//              if (rhsResult == ValidResult.RESULT_TRUE && isBarBar) {
//                // report error on else block: !e! || true
//                errorReporter.reportError(HintCode.DEAD_CODE, node.getRightOperand());
//                // only visit the RHS:
//                rhsCondition?.accept(this);
//                return null;
//              } else if (rhsResult == ValidResult.RESULT_FALSE && isAmpAmp) {
//                // report error on if block: !e! && false
//                errorReporter.reportError(HintCode.DEAD_CODE, node.getRightOperand());
//                // only visit the RHS:
//                rhsCondition?.accept(this);
//                return null;
//              }
//            }
    }
    return super.visitBinaryExpression(node);
  }

  /**
   * For each [Block], this method reports and error on all statements between the end of the
   * block and the first return statement (assuming there it is not at the end of the block.)
   *
   * @param node the block to evaluate
   */
  @override
  Object visitBlock(Block node) {
    NodeList<Statement> statements = node.statements;
    _checkForDeadStatementsInNodeList(statements);
    return null;
  }

  @override
  Object visitConditionalExpression(ConditionalExpression node) {
    Expression conditionExpression = node.condition;
    conditionExpression?.accept(this);
    if (!_isDebugConstant(conditionExpression)) {
      EvaluationResultImpl result =
          _getConstantBooleanValue(conditionExpression);
      if (result != null) {
        if (result.value.toBoolValue() == true) {
          // report error on else block: true ? 1 : !2!
          _errorReporter.reportErrorForNode(
              HintCode.DEAD_CODE, node.elseExpression);
          node.thenExpression?.accept(this);
          return null;
        } else {
          // report error on if block: false ? !1! : 2
          _errorReporter.reportErrorForNode(
              HintCode.DEAD_CODE, node.thenExpression);
          node.elseExpression?.accept(this);
          return null;
        }
      }
    }
    return super.visitConditionalExpression(node);
  }

  @override
  Object visitExportDirective(ExportDirective node) {
    ExportElement exportElement = node.element;
    if (exportElement != null) {
      // The element is null when the URI is invalid
      LibraryElement library = exportElement.exportedLibrary;
      if (library != null && !library.isSynthetic) {
        for (Combinator combinator in node.combinators) {
          _checkCombinator(exportElement.exportedLibrary, combinator);
        }
      }
    }
    return super.visitExportDirective(node);
  }

  @override
  Object visitIfStatement(IfStatement node) {
    Expression conditionExpression = node.condition;
    conditionExpression?.accept(this);
    if (!_isDebugConstant(conditionExpression)) {
      EvaluationResultImpl result =
          _getConstantBooleanValue(conditionExpression);
      if (result != null) {
        if (result.value.toBoolValue() == true) {
          // report error on else block: if(true) {} else {!}
          Statement elseStatement = node.elseStatement;
          if (elseStatement != null) {
            _errorReporter.reportErrorForNode(
                HintCode.DEAD_CODE, elseStatement);
            node.thenStatement?.accept(this);
            return null;
          }
        } else {
          // report error on if block: if (false) {!} else {}
          _errorReporter.reportErrorForNode(
              HintCode.DEAD_CODE, node.thenStatement);
          node.elseStatement?.accept(this);
          return null;
        }
      }
    }
    return super.visitIfStatement(node);
  }

  @override
  Object visitImportDirective(ImportDirective node) {
    ImportElement importElement = node.element;
    if (importElement != null) {
      // The element is null when the URI is invalid, but not when the URI is
      // valid but refers to a non-existent file.
      LibraryElement library = importElement.importedLibrary;
      if (library != null && !library.isSynthetic) {
        for (Combinator combinator in node.combinators) {
          _checkCombinator(library, combinator);
        }
      }
    }
    return super.visitImportDirective(node);
  }

  @override
  Object visitSwitchCase(SwitchCase node) {
    _checkForDeadStatementsInNodeList(node.statements, allowMandated: true);
    return super.visitSwitchCase(node);
  }

  @override
  Object visitSwitchDefault(SwitchDefault node) {
    _checkForDeadStatementsInNodeList(node.statements, allowMandated: true);
    return super.visitSwitchDefault(node);
  }

  @override
  Object visitTryStatement(TryStatement node) {
    node.body?.accept(this);
    node.finallyBlock?.accept(this);
    NodeList<CatchClause> catchClauses = node.catchClauses;
    int numOfCatchClauses = catchClauses.length;
    List<DartType> visitedTypes = new List<DartType>();
    for (int i = 0; i < numOfCatchClauses; i++) {
      CatchClause catchClause = catchClauses[i];
      if (catchClause.onKeyword != null) {
        // on-catch clause found, verify that the exception type is not a
        // subtype of a previous on-catch exception type
        DartType currentType = catchClause.exceptionType?.type;
        if (currentType != null) {
          if (currentType.isObject) {
            // Found catch clause clause that has Object as an exception type,
            // this is equivalent to having a catch clause that doesn't have an
            // exception type, visit the block, but generate an error on any
            // following catch clauses (and don't visit them).
            catchClause?.accept(this);
            if (i + 1 != numOfCatchClauses) {
              // this catch clause is not the last in the try statement
              CatchClause nextCatchClause = catchClauses[i + 1];
              CatchClause lastCatchClause = catchClauses[numOfCatchClauses - 1];
              int offset = nextCatchClause.offset;
              int length = lastCatchClause.end - offset;
              _errorReporter.reportErrorForOffset(
                  HintCode.DEAD_CODE_CATCH_FOLLOWING_CATCH, offset, length);
              return null;
            }
          }
          int length = visitedTypes.length;
          for (int j = 0; j < length; j++) {
            DartType type = visitedTypes[j];
            if (_typeSystem.isSubtypeOf(currentType, type)) {
              CatchClause lastCatchClause = catchClauses[numOfCatchClauses - 1];
              int offset = catchClause.offset;
              int length = lastCatchClause.end - offset;
              _errorReporter.reportErrorForOffset(
                  HintCode.DEAD_CODE_ON_CATCH_SUBTYPE,
                  offset,
                  length,
                  [currentType.displayName, type.displayName]);
              return null;
            }
          }
          visitedTypes.add(currentType);
        }
        catchClause?.accept(this);
      } else {
        // Found catch clause clause that doesn't have an exception type,
        // visit the block, but generate an error on any following catch clauses
        // (and don't visit them).
        catchClause?.accept(this);
        if (i + 1 != numOfCatchClauses) {
          // this catch clause is not the last in the try statement
          CatchClause nextCatchClause = catchClauses[i + 1];
          CatchClause lastCatchClause = catchClauses[numOfCatchClauses - 1];
          int offset = nextCatchClause.offset;
          int length = lastCatchClause.end - offset;
          _errorReporter.reportErrorForOffset(
              HintCode.DEAD_CODE_CATCH_FOLLOWING_CATCH, offset, length);
          return null;
        }
      }
    }
    return null;
  }

  @override
  Object visitWhileStatement(WhileStatement node) {
    Expression conditionExpression = node.condition;
    conditionExpression?.accept(this);
    if (!_isDebugConstant(conditionExpression)) {
      EvaluationResultImpl result =
          _getConstantBooleanValue(conditionExpression);
      if (result != null) {
        if (result.value.toBoolValue() == false) {
          // report error on if block: while (false) {!}
          _errorReporter.reportErrorForNode(HintCode.DEAD_CODE, node.body);
          return null;
        }
      }
    }
    node.body?.accept(this);
    return null;
  }

  /**
   * Resolve the names in the given [combinator] in the scope of the given
   * [library].
   */
  void _checkCombinator(LibraryElement library, Combinator combinator) {
    Namespace namespace =
        new NamespaceBuilder().createExportNamespaceForLibrary(library);
    NodeList<SimpleIdentifier> names;
    ErrorCode hintCode;
    if (combinator is HideCombinator) {
      names = combinator.hiddenNames;
      hintCode = HintCode.UNDEFINED_HIDDEN_NAME;
    } else {
      names = (combinator as ShowCombinator).shownNames;
      hintCode = HintCode.UNDEFINED_SHOWN_NAME;
    }
    for (SimpleIdentifier name in names) {
      String nameStr = name.name;
      Element element = namespace.get(nameStr);
      if (element == null) {
        element = namespace.get("$nameStr=");
      }
      if (element == null) {
        _errorReporter
            .reportErrorForNode(hintCode, name, [library.identifier, nameStr]);
      }
    }
  }

  /**
   * Given some [NodeList] of [Statement]s, from either a [Block] or
   * [SwitchMember], this loops through the list searching for dead statements.
   *
   * @param statements some ordered list of statements in a [Block] or [SwitchMember]
   * @param allowMandated allow dead statements mandated by the language spec.
   *            This allows for a final break, continue, return, or throw statement
   *            at the end of a switch case, that are mandated by the language spec.
   */
  void _checkForDeadStatementsInNodeList(NodeList<Statement> statements,
      {bool allowMandated: false}) {
    bool statementExits(Statement statement) {
      if (statement is BreakStatement) {
        return statement.label == null;
      } else if (statement is ContinueStatement) {
        return statement.label == null;
      }
      return ExitDetector.exits(statement);
    }

    int size = statements.length;
    for (int i = 0; i < size; i++) {
      Statement currentStatement = statements[i];
      currentStatement?.accept(this);
      if (statementExits(currentStatement) && i != size - 1) {
        Statement nextStatement = statements[i + 1];
        Statement lastStatement = statements[size - 1];
        // If mandated statements are allowed, and only the last statement is
        // dead, and it's a BreakStatement, then assume it is a statement
        // mandated by the language spec, there to avoid a
        // CASE_BLOCK_NOT_TERMINATED error.
        if (allowMandated && i == size - 2 && nextStatement is BreakStatement) {
          return;
        }
        int offset = nextStatement.offset;
        int length = lastStatement.end - offset;
        _errorReporter.reportErrorForOffset(HintCode.DEAD_CODE, offset, length);
        return;
      }
    }
  }

  /**
   * Given some [Expression], this method returns [ValidResult.RESULT_TRUE] if it is
   * `true`, [ValidResult.RESULT_FALSE] if it is `false`, or `null` if the
   * expression is not a constant boolean value.
   *
   * @param expression the expression to evaluate
   * @return [ValidResult.RESULT_TRUE] if it is `true`, [ValidResult.RESULT_FALSE]
   *         if it is `false`, or `null` if the expression is not a constant boolean
   *         value
   */
  EvaluationResultImpl _getConstantBooleanValue(Expression expression) {
    if (expression is BooleanLiteral) {
      if (expression.value) {
        return new EvaluationResultImpl(
            new DartObjectImpl(null, BoolState.from(true)));
      } else {
        return new EvaluationResultImpl(
            new DartObjectImpl(null, BoolState.from(false)));
      }
    }
    // Don't consider situations where we could evaluate to a constant boolean
    // expression with the ConstantVisitor
    // else {
    // EvaluationResultImpl result = expression.accept(new ConstantVisitor());
    // if (result == ValidResult.RESULT_TRUE) {
    // return ValidResult.RESULT_TRUE;
    // } else if (result == ValidResult.RESULT_FALSE) {
    // return ValidResult.RESULT_FALSE;
    // }
    // return null;
    // }
    return null;
  }

  /**
   * Return `true` if and only if the passed expression is resolved to a constant variable.
   *
   * @param expression some conditional expression
   * @return `true` if and only if the passed expression is resolved to a constant variable
   */
  bool _isDebugConstant(Expression expression) {
    Element element = null;
    if (expression is Identifier) {
      element = expression.staticElement;
    } else if (expression is PropertyAccess) {
      element = expression.propertyName.staticElement;
    }
    if (element is PropertyAccessorElement) {
      PropertyInducingElement variable = element.variable;
      return variable != null && variable.isConst;
    }
    return false;
  }
}

/**
 * A visitor that resolves directives in an AST structure to already built
 * elements.
 *
 * The resulting AST must have everything resolved that would have been resolved
 * by a [DirectiveElementBuilder].
 */
class DirectiveResolver extends SimpleAstVisitor {
  final Map<Source, int> sourceModificationTimeMap;
  final Map<Source, SourceKind> importSourceKindMap;
  final Map<Source, SourceKind> exportSourceKindMap;
  final List<AnalysisError> errors = <AnalysisError>[];

  LibraryElement _enclosingLibrary;

  DirectiveResolver(this.sourceModificationTimeMap, this.importSourceKindMap,
      this.exportSourceKindMap);

  @override
  void visitCompilationUnit(CompilationUnit node) {
    _enclosingLibrary =
        resolutionMap.elementDeclaredByCompilationUnit(node).library;
    for (Directive directive in node.directives) {
      directive.accept(this);
    }
  }

  @override
  void visitExportDirective(ExportDirective node) {
    int nodeOffset = node.offset;
    node.element = null;
    for (ExportElement element in _enclosingLibrary.exports) {
      if (element.nameOffset == nodeOffset) {
        node.element = element;
        // Verify the exported source kind.
        Source exportedSource = element.exportedLibrary.source;
        int exportedTime = sourceModificationTimeMap[exportedSource] ?? -1;
        if (exportedTime >= 0 &&
            exportSourceKindMap[exportedSource] != SourceKind.LIBRARY) {
          StringLiteral uriLiteral = node.uri;
          errors.add(new AnalysisError(
              _enclosingLibrary.source,
              uriLiteral.offset,
              uriLiteral.length,
              CompileTimeErrorCode.EXPORT_OF_NON_LIBRARY,
              [uriLiteral.toSource()]));
        }
        break;
      }
    }
  }

  @override
  void visitImportDirective(ImportDirective node) {
    int nodeOffset = node.offset;
    node.element = null;
    for (ImportElement element in _enclosingLibrary.imports) {
      if (element.nameOffset == nodeOffset) {
        node.element = element;
        // Verify the imported source kind.
        Source importedSource = element.importedLibrary.source;
        int importedTime = sourceModificationTimeMap[importedSource] ?? -1;
        if (importedTime >= 0 &&
            importSourceKindMap[importedSource] != SourceKind.LIBRARY) {
          StringLiteral uriLiteral = node.uri;
          ErrorCode errorCode = element.isDeferred
              ? StaticWarningCode.IMPORT_OF_NON_LIBRARY
              : CompileTimeErrorCode.IMPORT_OF_NON_LIBRARY;
          errors.add(new AnalysisError(
              _enclosingLibrary.source,
              uriLiteral.offset,
              uriLiteral.length,
              errorCode,
              [uriLiteral.toSource()]));
        }
        break;
      }
    }
  }

  @override
  void visitLibraryDirective(LibraryDirective node) {
    node.element = _enclosingLibrary;
  }
}

/**
 * Instances of the class `ElementHolder` hold on to elements created while traversing an AST
 * structure so that they can be accessed when creating their enclosing element.
 */
class ElementHolder {
  List<PropertyAccessorElement> _accessors;

  List<ConstructorElement> _constructors;

  List<ClassElement> _enums;

  List<FieldElement> _fields;

  List<FunctionElement> _functions;

  List<LabelElement> _labels;

  List<LocalVariableElement> _localVariables;

  List<MethodElement> _methods;

  List<ParameterElement> _parameters;

  List<TopLevelVariableElement> _topLevelVariables;

  List<ClassElement> _types;

  List<FunctionTypeAliasElement> _typeAliases;

  List<TypeParameterElement> _typeParameters;

  List<PropertyAccessorElement> get accessors {
    if (_accessors == null) {
      return PropertyAccessorElement.EMPTY_LIST;
    }
    List<PropertyAccessorElement> result = _accessors;
    _accessors = null;
    return result;
  }

  List<ConstructorElement> get constructors {
    if (_constructors == null) {
      return ConstructorElement.EMPTY_LIST;
    }
    List<ConstructorElement> result = _constructors;
    _constructors = null;
    return result;
  }

  List<ClassElement> get enums {
    if (_enums == null) {
      return ClassElement.EMPTY_LIST;
    }
    List<ClassElement> result = _enums;
    _enums = null;
    return result;
  }

  List<FieldElement> get fields {
    if (_fields == null) {
      return FieldElement.EMPTY_LIST;
    }
    List<FieldElement> result = _fields;
    _fields = null;
    return result;
  }

  List<FieldElement> get fieldsWithoutFlushing {
    if (_fields == null) {
      return FieldElement.EMPTY_LIST;
    }
    List<FieldElement> result = _fields;
    return result;
  }

  List<FunctionElement> get functions {
    if (_functions == null) {
      return FunctionElement.EMPTY_LIST;
    }
    List<FunctionElement> result = _functions;
    _functions = null;
    return result;
  }

  List<LabelElement> get labels {
    if (_labels == null) {
      return LabelElement.EMPTY_LIST;
    }
    List<LabelElement> result = _labels;
    _labels = null;
    return result;
  }

  List<LocalVariableElement> get localVariables {
    if (_localVariables == null) {
      return LocalVariableElement.EMPTY_LIST;
    }
    List<LocalVariableElement> result = _localVariables;
    _localVariables = null;
    return result;
  }

  List<MethodElement> get methods {
    if (_methods == null) {
      return MethodElement.EMPTY_LIST;
    }
    List<MethodElement> result = _methods;
    _methods = null;
    return result;
  }

  List<ParameterElement> get parameters {
    if (_parameters == null) {
      return ParameterElement.EMPTY_LIST;
    }
    List<ParameterElement> result = _parameters;
    _parameters = null;
    return result;
  }

  List<TopLevelVariableElement> get topLevelVariables {
    if (_topLevelVariables == null) {
      return TopLevelVariableElement.EMPTY_LIST;
    }
    List<TopLevelVariableElement> result = _topLevelVariables;
    _topLevelVariables = null;
    return result;
  }

  List<FunctionTypeAliasElement> get typeAliases {
    if (_typeAliases == null) {
      return FunctionTypeAliasElement.EMPTY_LIST;
    }
    List<FunctionTypeAliasElement> result = _typeAliases;
    _typeAliases = null;
    return result;
  }

  List<TypeParameterElement> get typeParameters {
    if (_typeParameters == null) {
      return TypeParameterElement.EMPTY_LIST;
    }
    List<TypeParameterElement> result = _typeParameters;
    _typeParameters = null;
    return result;
  }

  List<ClassElement> get types {
    if (_types == null) {
      return ClassElement.EMPTY_LIST;
    }
    List<ClassElement> result = _types;
    _types = null;
    return result;
  }

  void addAccessor(PropertyAccessorElement element) {
    if (_accessors == null) {
      _accessors = new List<PropertyAccessorElement>();
    }
    _accessors.add(element);
  }

  void addConstructor(ConstructorElement element) {
    if (_constructors == null) {
      _constructors = new List<ConstructorElement>();
    }
    _constructors.add(element);
  }

  void addEnum(ClassElement element) {
    if (_enums == null) {
      _enums = new List<ClassElement>();
    }
    _enums.add(element);
  }

  void addField(FieldElement element) {
    if (_fields == null) {
      _fields = new List<FieldElement>();
    }
    _fields.add(element);
  }

  void addFunction(FunctionElement element) {
    if (_functions == null) {
      _functions = new List<FunctionElement>();
    }
    _functions.add(element);
  }

  void addLabel(LabelElement element) {
    if (_labels == null) {
      _labels = new List<LabelElement>();
    }
    _labels.add(element);
  }

  void addLocalVariable(LocalVariableElement element) {
    if (_localVariables == null) {
      _localVariables = new List<LocalVariableElement>();
    }
    _localVariables.add(element);
  }

  void addMethod(MethodElement element) {
    if (_methods == null) {
      _methods = new List<MethodElement>();
    }
    _methods.add(element);
  }

  void addParameter(ParameterElement element) {
    if (_parameters == null) {
      _parameters = new List<ParameterElement>();
    }
    _parameters.add(element);
  }

  void addTopLevelVariable(TopLevelVariableElement element) {
    if (_topLevelVariables == null) {
      _topLevelVariables = new List<TopLevelVariableElement>();
    }
    _topLevelVariables.add(element);
  }

  void addType(ClassElement element) {
    if (_types == null) {
      _types = new List<ClassElement>();
    }
    _types.add(element);
  }

  void addTypeAlias(FunctionTypeAliasElement element) {
    if (_typeAliases == null) {
      _typeAliases = new List<FunctionTypeAliasElement>();
    }
    _typeAliases.add(element);
  }

  void addTypeParameter(TypeParameterElement element) {
    if (_typeParameters == null) {
      _typeParameters = new List<TypeParameterElement>();
    }
    _typeParameters.add(element);
  }

  FieldElement getField(String fieldName, {bool synthetic: false}) {
    if (_fields == null) {
      return null;
    }
    int length = _fields.length;
    for (int i = 0; i < length; i++) {
      FieldElement field = _fields[i];
      if (field.name == fieldName && field.isSynthetic == synthetic) {
        return field;
      }
    }
    return null;
  }

  TopLevelVariableElement getTopLevelVariable(String variableName) {
    if (_topLevelVariables == null) {
      return null;
    }
    int length = _topLevelVariables.length;
    for (int i = 0; i < length; i++) {
      TopLevelVariableElement variable = _topLevelVariables[i];
      if (variable.name == variableName) {
        return variable;
      }
    }
    return null;
  }

  void validate() {
    StringBuffer buffer = new StringBuffer();
    if (_accessors != null) {
      buffer.write(_accessors.length);
      buffer.write(" accessors");
    }
    if (_constructors != null) {
      if (buffer.length > 0) {
        buffer.write("; ");
      }
      buffer.write(_constructors.length);
      buffer.write(" constructors");
    }
    if (_fields != null) {
      if (buffer.length > 0) {
        buffer.write("; ");
      }
      buffer.write(_fields.length);
      buffer.write(" fields");
    }
    if (_functions != null) {
      if (buffer.length > 0) {
        buffer.write("; ");
      }
      buffer.write(_functions.length);
      buffer.write(" functions");
    }
    if (_labels != null) {
      if (buffer.length > 0) {
        buffer.write("; ");
      }
      buffer.write(_labels.length);
      buffer.write(" labels");
    }
    if (_localVariables != null) {
      if (buffer.length > 0) {
        buffer.write("; ");
      }
      buffer.write(_localVariables.length);
      buffer.write(" local variables");
    }
    if (_methods != null) {
      if (buffer.length > 0) {
        buffer.write("; ");
      }
      buffer.write(_methods.length);
      buffer.write(" methods");
    }
    if (_parameters != null) {
      if (buffer.length > 0) {
        buffer.write("; ");
      }
      buffer.write(_parameters.length);
      buffer.write(" parameters");
    }
    if (_topLevelVariables != null) {
      if (buffer.length > 0) {
        buffer.write("; ");
      }
      buffer.write(_topLevelVariables.length);
      buffer.write(" top-level variables");
    }
    if (_types != null) {
      if (buffer.length > 0) {
        buffer.write("; ");
      }
      buffer.write(_types.length);
      buffer.write(" types");
    }
    if (_typeAliases != null) {
      if (buffer.length > 0) {
        buffer.write("; ");
      }
      buffer.write(_typeAliases.length);
      buffer.write(" type aliases");
    }
    if (_typeParameters != null) {
      if (buffer.length > 0) {
        buffer.write("; ");
      }
      buffer.write(_typeParameters.length);
      buffer.write(" type parameters");
    }
    if (buffer.length > 0) {
      AnalysisEngine.instance.logger
          .logError("Failed to capture elements: $buffer");
    }
  }
}

/**
 * Instances of the class `EnumMemberBuilder` build the members in enum declarations.
 */
class EnumMemberBuilder extends RecursiveAstVisitor<Object> {
  /**
   * The type provider used to access the types needed to build an element model for enum
   * declarations.
   */
  final TypeProvider _typeProvider;

  /**
   * Initialize a newly created enum member builder.
   *
   * @param typeProvider the type provider used to access the types needed to build an element model
   *          for enum declarations
   */
  EnumMemberBuilder(this._typeProvider);

  @override
  Object visitEnumDeclaration(EnumDeclaration node) {
    //
    // Finish building the enum.
    //
    EnumElementImpl enumElement = node.name.staticElement as EnumElementImpl;
    InterfaceType enumType = enumElement.type;
    //
    // Populate the fields.
    //
    List<FieldElement> fields = new List<FieldElement>();
    List<PropertyAccessorElement> getters = new List<PropertyAccessorElement>();
    InterfaceType intType = _typeProvider.intType;
    String indexFieldName = "index";
    FieldElementImpl indexField = new FieldElementImpl(indexFieldName, -1);
    indexField.isFinal = true;
    indexField.isSynthetic = true;
    indexField.type = intType;
    fields.add(indexField);
    getters.add(_createGetter(indexField));
    ConstFieldElementImpl valuesField = new ConstFieldElementImpl("values", -1);
    valuesField.isStatic = true;
    valuesField.isConst = true;
    valuesField.isSynthetic = true;
    valuesField.type = _typeProvider.listType.instantiate(<DartType>[enumType]);
    fields.add(valuesField);
    getters.add(_createGetter(valuesField));
    //
    // Build the enum constants.
    //
    NodeList<EnumConstantDeclaration> constants = node.constants;
    List<DartObjectImpl> constantValues = new List<DartObjectImpl>();
    int constantCount = constants.length;
    for (int i = 0; i < constantCount; i++) {
      EnumConstantDeclaration constant = constants[i];
      FieldElementImpl constantField = constant.name.staticElement;
      //
      // Create a value for the constant.
      //
      HashMap<String, DartObjectImpl> fieldMap =
          new HashMap<String, DartObjectImpl>();
      fieldMap[indexFieldName] = new DartObjectImpl(intType, new IntState(i));
      DartObjectImpl value =
          new DartObjectImpl(enumType, new GenericState(fieldMap));
      constantValues.add(value);
      constantField.evaluationResult = new EvaluationResultImpl(value);
      fields.add(constantField);
      getters.add(constantField.getter);
    }
    //
    // Build the value of the 'values' field.
    //
    valuesField.evaluationResult = new EvaluationResultImpl(
        new DartObjectImpl(valuesField.type, new ListState(constantValues)));
    //
    // Finish building the enum.
    //
    enumElement.fields = fields;
    enumElement.accessors = getters;
    // Client code isn't allowed to invoke the constructor, so we do not model
    // it.
    return super.visitEnumDeclaration(node);
  }

  /**
   * Create a getter that corresponds to the given [field].
   */
  PropertyAccessorElement _createGetter(FieldElementImpl field) {
    return new PropertyAccessorElementImpl_ImplicitGetter(field);
  }
}

/**
 * Instances of the class `ExitDetector` determine whether the visited AST node is guaranteed
 * to terminate by executing a `return` statement, `throw` expression, `rethrow`
 * expression, or simple infinite loop such as `while(true)`.
 */
class ExitDetector extends GeneralizingAstVisitor<bool> {
  /**
   * Set to `true` when a `break` is encountered, and reset to `false` when a
   * `do`, `while`, `for` or `switch` block is entered.
   */
  bool _enclosingBlockContainsBreak = false;

  /**
   * Set to `true` when a `continue` is encountered, and reset to `false` when a
   * `do`, `while`, `for` or `switch` block is entered.
   */
  bool _enclosingBlockContainsContinue = false;

  /**
   * Add node when a labelled `break` is encountered.
   */
  Set<AstNode> _enclosingBlockBreaksLabel = new Set<AstNode>();

  @override
  bool visitArgumentList(ArgumentList node) =>
      _visitExpressions(node.arguments);

  @override
  bool visitAsExpression(AsExpression node) => _nodeExits(node.expression);

  @override
  bool visitAssertInitializer(AssertInitializer node) => false;

  @override
  bool visitAssertStatement(AssertStatement node) => false;

  @override
  bool visitAssignmentExpression(AssignmentExpression node) {
    Expression leftHandSide = node.leftHandSide;
    if (_nodeExits(leftHandSide)) {
      return true;
    }
    TokenType operatorType = node.operator.type;
    if (operatorType == TokenType.AMPERSAND_AMPERSAND_EQ ||
        operatorType == TokenType.BAR_BAR_EQ ||
        operatorType == TokenType.QUESTION_QUESTION_EQ) {
      return false;
    }
    if (leftHandSide is PropertyAccess &&
        leftHandSide.operator.type == TokenType.QUESTION_PERIOD) {
      return false;
    }
    return _nodeExits(node.rightHandSide);
  }

  @override
  bool visitAwaitExpression(AwaitExpression node) =>
      _nodeExits(node.expression);

  @override
  bool visitBinaryExpression(BinaryExpression node) {
    Expression lhsExpression = node.leftOperand;
    Expression rhsExpression = node.rightOperand;
    TokenType operatorType = node.operator.type;
    // If the operator is ||, then only consider the RHS of the binary
    // expression if the left hand side is the false literal.
    // TODO(jwren) Do we want to take constant expressions into account,
    // evaluate if(false) {} differently than if(<condition>), when <condition>
    // evaluates to a constant false value?
    if (operatorType == TokenType.BAR_BAR) {
      if (lhsExpression is BooleanLiteral) {
        if (!lhsExpression.value) {
          return _nodeExits(rhsExpression);
        }
      }
      return _nodeExits(lhsExpression);
    }
    // If the operator is &&, then only consider the RHS of the binary
    // expression if the left hand side is the true literal.
    if (operatorType == TokenType.AMPERSAND_AMPERSAND) {
      if (lhsExpression is BooleanLiteral) {
        if (lhsExpression.value) {
          return _nodeExits(rhsExpression);
        }
      }
      return _nodeExits(lhsExpression);
    }
    // If the operator is ??, then don't consider the RHS of the binary
    // expression.
    if (operatorType == TokenType.QUESTION_QUESTION) {
      return _nodeExits(lhsExpression);
    }
    return _nodeExits(lhsExpression) || _nodeExits(rhsExpression);
  }

  @override
  bool visitBlock(Block node) => _visitStatements(node.statements);

  @override
  bool visitBlockFunctionBody(BlockFunctionBody node) => _nodeExits(node.block);

  @override
  bool visitBreakStatement(BreakStatement node) {
    _enclosingBlockContainsBreak = true;
    if (node.label != null) {
      _enclosingBlockBreaksLabel.add(node.target);
    }
    return false;
  }

  @override
  bool visitCascadeExpression(CascadeExpression node) =>
      _nodeExits(node.target) || _visitExpressions(node.cascadeSections);

  @override
  bool visitConditionalExpression(ConditionalExpression node) {
    Expression conditionExpression = node.condition;
    Expression thenStatement = node.thenExpression;
    Expression elseStatement = node.elseExpression;
    // TODO(jwren) Do we want to take constant expressions into account,
    // evaluate if(false) {} differently than if(<condition>), when <condition>
    // evaluates to a constant false value?
    if (_nodeExits(conditionExpression)) {
      return true;
    }
    if (thenStatement == null || elseStatement == null) {
      return false;
    }
    return thenStatement.accept(this) && elseStatement.accept(this);
  }

  @override
  bool visitContinueStatement(ContinueStatement node) {
    _enclosingBlockContainsContinue = true;
    return false;
  }

  @override
  bool visitDoStatement(DoStatement node) {
    bool outerBreakValue = _enclosingBlockContainsBreak;
    bool outerContinueValue = _enclosingBlockContainsContinue;
    _enclosingBlockContainsBreak = false;
    _enclosingBlockContainsContinue = false;
    try {
      bool bodyExits = _nodeExits(node.body);
      bool containsBreakOrContinue =
          _enclosingBlockContainsBreak || _enclosingBlockContainsContinue;
      // Even if we determine that the body "exits", there might be break or
      // continue statements that actually mean it _doesn't_ always exit.
      if (bodyExits && !containsBreakOrContinue) {
        return true;
      }