pkg/analysis_server/lib/src/services/correction/base_processor.dart

// Copyright (c) 2019, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

import 'package:analysis_server/src/services/correction/util.dart';
import 'package:analysis_server/src/utilities/flutter.dart';
import 'package:analyzer/dart/analysis/results.dart';
import 'package:analyzer/dart/analysis/session.dart';
import 'package:analyzer/dart/ast/ast.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/source/source_range.dart';
import 'package:analyzer/src/dart/analysis/experiments.dart';
import 'package:analyzer/src/dart/analysis/session_helper.dart';
import 'package:analyzer/src/dart/ast/utilities.dart';
import 'package:analyzer/src/generated/engine.dart' show AnalysisOptionsImpl;
import 'package:analyzer/src/generated/resolver.dart';
import 'package:analyzer_plugin/src/utilities/change_builder/change_builder_dart.dart';
import 'package:analyzer_plugin/utilities/change_builder/change_builder_core.dart';
import 'package:analyzer_plugin/utilities/change_builder/change_builder_dart.dart';
import 'package:analyzer_plugin/utilities/change_builder/change_workspace.dart';
import 'package:analyzer_plugin/utilities/range_factory.dart';
import 'package:meta/meta.dart';

/// Base class for common processor functionality.
abstract class BaseProcessor {
  final int selectionOffset;
  final int selectionLength;
  final int selectionEnd;

  final CorrectionUtils utils;
  final String file;

  final TypeProvider typeProvider;
  final Flutter flutter;

  final AnalysisSession session;
  final AnalysisSessionHelper sessionHelper;
  final ResolvedUnitResult resolvedResult;
  final ChangeWorkspace workspace;

  AstNode node;

  BaseProcessor({
    this.selectionOffset = -1,
    this.selectionLength = 0,
    @required this.resolvedResult,
    @required this.workspace,
  })  : file = resolvedResult.path,
        flutter = Flutter.of(resolvedResult),
        session = resolvedResult.session,
        sessionHelper = AnalysisSessionHelper(resolvedResult.session),
        typeProvider = resolvedResult.typeProvider,
        selectionEnd = (selectionOffset ?? 0) + (selectionLength ?? 0),
        utils = CorrectionUtils(resolvedResult);

  /// Returns the EOL to use for this [CompilationUnit].
  String get eol => utils.endOfLine;

  /// Return the status of known experiments.
  ExperimentStatus get experimentStatus =>
      (session.analysisContext.analysisOptions as AnalysisOptionsImpl)
          .experimentStatus;

  Future<ChangeBuilder>
      createBuilder_addTypeAnnotation_DeclaredIdentifier() async {
    DeclaredIdentifier declaredIdentifier =
        node.thisOrAncestorOfType<DeclaredIdentifier>();
    if (declaredIdentifier == null) {
      ForStatement forEach = node.thisOrAncestorMatching(
          (node) => node is ForStatement && node.forLoopParts is ForEachParts);
      ForEachParts forEachParts = forEach?.forLoopParts;
      int offset = node.offset;
      if (forEach != null &&
          forEachParts.iterable != null &&
          offset < forEachParts.iterable.offset) {
        declaredIdentifier = forEachParts is ForEachPartsWithDeclaration
            ? forEachParts.loopVariable
            : null;
      }
    }
    if (declaredIdentifier == null) {
      _coverageMarker();
      return null;
    }
    // Ensure that there isn't already a type annotation.
    if (declaredIdentifier.type != null) {
      _coverageMarker();
      return null;
    }
    DartType type = declaredIdentifier.identifier.staticType;
    if (type is! InterfaceType && type is! FunctionType) {
      _coverageMarker();
      return null;
    }
    _configureTargetLocation(node);

    var changeBuilder = _newDartChangeBuilder();
    bool validChange = true;
    await changeBuilder.addFileEdit(file, (DartFileEditBuilder builder) {
      Token keyword = declaredIdentifier.keyword;
      if (keyword.keyword == Keyword.VAR) {
        builder.addReplacement(range.token(keyword), (DartEditBuilder builder) {
          validChange = builder.writeType(type);
        });
      } else {
        builder.addInsertion(declaredIdentifier.identifier.offset,
            (DartEditBuilder builder) {
          validChange = builder.writeType(type);
          builder.write(' ');
        });
      }
    });
    return validChange ? changeBuilder : null;
  }

  Future<ChangeBuilder>
      createBuilder_addTypeAnnotation_SimpleFormalParameter() async {
    AstNode node = this.node;
    // should be the name of a simple parameter
    if (node is! SimpleIdentifier || node.parent is! SimpleFormalParameter) {
      _coverageMarker();
      return null;
    }
    SimpleIdentifier name = node;
    SimpleFormalParameter parameter = node.parent;
    // the parameter should not have a type
    if (parameter.type != null) {
      _coverageMarker();
      return null;
    }
    // prepare the type
    DartType type = parameter.declaredElement.type;
    // TODO(scheglov) If the parameter is in a method declaration, and if the
    // method overrides a method that has a type for the corresponding
    // parameter, it would be nice to copy down the type from the overridden
    // method.
    if (type is! InterfaceType) {
      _coverageMarker();
      return null;
    }
    // prepare type source
    _configureTargetLocation(node);

    var changeBuilder = _newDartChangeBuilder();
    bool validChange = true;
    await changeBuilder.addFileEdit(file, (DartFileEditBuilder builder) {
      builder.addInsertion(name.offset, (DartEditBuilder builder) {
        validChange = builder.writeType(type);
        builder.write(' ');
      });
    });
    return validChange ? changeBuilder : null;
  }

  Future<ChangeBuilder> createBuilder_convertToNullAware() async {
    AstNode node = this.node;
    if (node is! ConditionalExpression) {
      _coverageMarker();
      return null;
    }
    ConditionalExpression conditional = node;
    Expression condition = conditional.condition.unParenthesized;
    SimpleIdentifier identifier;
    Expression nullExpression;
    Expression nonNullExpression;
    int periodOffset;

    if (condition is BinaryExpression) {
      //
      // Identify the variable being compared to `null`, or return if the
      // condition isn't a simple comparison of `null` to a variable's value.
      //
      Expression leftOperand = condition.leftOperand;
      Expression rightOperand = condition.rightOperand;
      if (leftOperand is NullLiteral && rightOperand is SimpleIdentifier) {
        identifier = rightOperand;
      } else if (rightOperand is NullLiteral &&
          leftOperand is SimpleIdentifier) {
        identifier = leftOperand;
      } else {
        _coverageMarker();
        return null;
      }
      if (identifier.staticElement is! LocalElement) {
        _coverageMarker();
        return null;
      }
      //
      // Identify the expression executed when the variable is `null` and when
      // it is non-`null`. Return if the `null` expression isn't a null literal
      // or if the non-`null` expression isn't a method invocation whose target
      // is the save variable being compared to `null`.
      //
      if (condition.operator.type == TokenType.EQ_EQ) {
        nullExpression = conditional.thenExpression;
        nonNullExpression = conditional.elseExpression;
      } else if (condition.operator.type == TokenType.BANG_EQ) {
        nonNullExpression = conditional.thenExpression;
        nullExpression = conditional.elseExpression;
      }
      if (nullExpression == null || nonNullExpression == null) {
        _coverageMarker();
        return null;
      }
      if (nullExpression.unParenthesized is! NullLiteral) {
        _coverageMarker();
        return null;
      }
      Expression unwrappedExpression = nonNullExpression.unParenthesized;
      Expression target;
      Token operator;
      if (unwrappedExpression is MethodInvocation) {
        target = unwrappedExpression.target;
        operator = unwrappedExpression.operator;
      } else if (unwrappedExpression is PrefixedIdentifier) {
        target = unwrappedExpression.prefix;
        operator = unwrappedExpression.period;
      } else {
        _coverageMarker();
        return null;
      }
      if (operator.type != TokenType.PERIOD) {
        _coverageMarker();
        return null;
      }
      if (!(target is SimpleIdentifier &&
          target.staticElement == identifier.staticElement)) {
        _coverageMarker();
        return null;
      }
      periodOffset = operator.offset;

      DartChangeBuilder changeBuilder = _newDartChangeBuilder();
      await changeBuilder.addFileEdit(file, (DartFileEditBuilder builder) {
        builder.addDeletion(range.startStart(node, nonNullExpression));
        builder.addSimpleInsertion(periodOffset, '?');
        builder.addDeletion(range.endEnd(nonNullExpression, node));
      });
      return changeBuilder;
    }
    return null;
  }

  Future<ChangeBuilder> createBuilder_convertToExpressionFunctionBody() async {
    // prepare current body
    FunctionBody body = getEnclosingFunctionBody();
    if (body is! BlockFunctionBody || body.isGenerator) {
      _coverageMarker();
      return null;
    }
    // prepare return statement
    List<Statement> statements = (body as BlockFunctionBody).block.statements;
    if (statements.length != 1) {
      _coverageMarker();
      return null;
    }
    Statement onlyStatement = statements.first;
    // prepare returned expression
    Expression returnExpression;
    if (onlyStatement is ReturnStatement) {
      returnExpression = onlyStatement.expression;
    } else if (onlyStatement is ExpressionStatement) {
      returnExpression = onlyStatement.expression;
    }
    if (returnExpression == null) {
      _coverageMarker();
      return null;
    }

    // Return expressions can be quite large, e.g. Flutter build() methods.
    // It is surprising to see this Quick Assist deep in the function body.
    if (selectionOffset >= returnExpression.offset) {
      _coverageMarker();
      return null;
    }

    var changeBuilder = _newDartChangeBuilder();
    await changeBuilder.addFileEdit(file, (DartFileEditBuilder builder) {
      builder.addReplacement(range.node(body), (DartEditBuilder builder) {
        if (body.isAsynchronous) {
          builder.write('async ');
        }
        builder.write('=> ');
        builder.write(_getNodeText(returnExpression));
        if (body.parent is! FunctionExpression ||
            body.parent.parent is FunctionDeclaration) {
          builder.write(';');
        }
      });
    });
    return changeBuilder;
  }

  /// Returns the text of the given node in the unit.
  String /* TODO (pq): make visible */ _getNodeText(AstNode node) =>
      utils.getNodeText(node);

  FunctionBody getEnclosingFunctionBody() {
    // TODO(brianwilkerson) Determine whether there is a reason why this method
    // isn't just "return node.getAncestor((node) => node is FunctionBody);"
    {
      FunctionExpression function =
          node.thisOrAncestorOfType<FunctionExpression>();
      if (function != null) {
        return function.body;
      }
    }
    {
      FunctionDeclaration function =
          node.thisOrAncestorOfType<FunctionDeclaration>();
      if (function != null) {
        return function.functionExpression.body;
      }
    }
    {
      ConstructorDeclaration constructor =
          node.thisOrAncestorOfType<ConstructorDeclaration>();
      if (constructor != null) {
        return constructor.body;
      }
    }
    {
      MethodDeclaration method = node.thisOrAncestorOfType<MethodDeclaration>();
      if (method != null) {
        return method.body;
      }
    }
    return null;
  }

  Future<ChangeBuilder>
      createBuilder_addTypeAnnotation_VariableDeclaration() async {
    AstNode node = this.node;
    // prepare VariableDeclarationList
    VariableDeclarationList declarationList =
        node.thisOrAncestorOfType<VariableDeclarationList>();
    if (declarationList == null) {
      _coverageMarker();
      return null;
    }
    // may be has type annotation already
    if (declarationList.type != null) {
      _coverageMarker();
      return null;
    }
    // prepare single VariableDeclaration
    List<VariableDeclaration> variables = declarationList.variables;
    if (variables.length != 1) {
      _coverageMarker();
      return null;
    }
    VariableDeclaration variable = variables[0];
    // must be not after the name of the variable
    if (selectionOffset > variable.name.end) {
      _coverageMarker();
      return null;
    }
    // we need an initializer to get the type from
    Expression initializer = variable.initializer;
    if (initializer == null) {
      _coverageMarker();
      return null;
    }
    DartType type = initializer.staticType;
    // prepare type source
    if ((type is! InterfaceType || type.isDartCoreNull) &&
        type is! FunctionType) {
      _coverageMarker();
      return null;
    }
    _configureTargetLocation(node);

    var changeBuilder = _newDartChangeBuilder();
    bool validChange = true;
    await changeBuilder.addFileEdit(file, (DartFileEditBuilder builder) {
      Token keyword = declarationList.keyword;
      if (keyword?.keyword == Keyword.VAR) {
        builder.addReplacement(range.token(keyword), (DartEditBuilder builder) {
          validChange = builder.writeType(type);
        });
      } else {
        builder.addInsertion(variable.offset, (DartEditBuilder builder) {
          validChange = builder.writeType(type);
          builder.write(' ');
        });
      }
    });
    return validChange ? changeBuilder : null;
  }

  Future<ChangeBuilder>
      createBuilder_convertConditionalExpressionToIfElement() async {
    AstNode node = this.node.thisOrAncestorOfType<ConditionalExpression>();
    if (node == null) {
      _coverageMarker();
      return null;
    }
    AstNode nodeToReplace = node;
    AstNode parent = node.parent;
    while (parent is ParenthesizedExpression) {
      nodeToReplace = parent;
      parent = parent.parent;
    }
    if (parent is ListLiteral || (parent is SetOrMapLiteral && parent.isSet)) {
      ConditionalExpression conditional = node;
      Expression condition = conditional.condition.unParenthesized;
      Expression thenExpression = conditional.thenExpression.unParenthesized;
      Expression elseExpression = conditional.elseExpression.unParenthesized;

      DartChangeBuilder changeBuilder = _newDartChangeBuilder();
      await changeBuilder.addFileEdit(file, (DartFileEditBuilder builder) {
        builder.addReplacement(range.node(nodeToReplace),
            (DartEditBuilder builder) {
          builder.write('if (');
          builder.write(utils.getNodeText(condition));
          builder.write(') ');
          builder.write(utils.getNodeText(thenExpression));
          builder.write(' else ');
          builder.write(utils.getNodeText(elseExpression));
        });
      });
      return changeBuilder;
    }

    return null;
  }

  Future<ChangeBuilder> createBuilder_convertDocumentationIntoLine() async {
    Comment comment = node.thisOrAncestorOfType<Comment>();
    if (comment == null ||
        !comment.isDocumentation ||
        comment.tokens.length != 1) {
      _coverageMarker();
      return null;
    }
    Token token = comment.tokens.first;
    if (token.type != TokenType.MULTI_LINE_COMMENT) {
      _coverageMarker();
      return null;
    }
    String text = token.lexeme;
    List<String> lines = text.split('\n');
    String prefix = utils.getNodePrefix(comment);
    List<String> newLines = <String>[];
    bool firstLine = true;
    String linePrefix = '';
    for (String line in lines) {
      if (firstLine) {
        firstLine = false;
        String expectedPrefix = '/**';
        if (!line.startsWith(expectedPrefix)) {
          _coverageMarker();
          return null;
        }
        line = line.substring(expectedPrefix.length).trim();
        if (line.isNotEmpty) {
          newLines.add('/// $line');
          linePrefix = eol + prefix;
        }
      } else {
        if (line.startsWith(prefix + ' */')) {
          break;
        }
        String expectedPrefix = prefix + ' *';
        if (!line.startsWith(expectedPrefix)) {
          _coverageMarker();
          return null;
        }
        line = line.substring(expectedPrefix.length);
        if (line.isEmpty) {
          newLines.add('$linePrefix///');
        } else {
          newLines.add('$linePrefix///$line');
        }
        linePrefix = eol + prefix;
      }
    }

    var changeBuilder = _newDartChangeBuilder();
    await changeBuilder.addFileEdit(file, (DartFileEditBuilder builder) {
      builder.addReplacement(range.node(comment), (DartEditBuilder builder) {
        for (String newLine in newLines) {
          builder.write(newLine);
        }
      });
    });
    return changeBuilder;
  }

  Future<ChangeBuilder>
      createBuilder_convertMapFromIterableToForLiteral() async {
    //
    // Ensure that the selection is inside an invocation of Map.fromIterable.
    //
    InstanceCreationExpression creation =
        node.thisOrAncestorOfType<InstanceCreationExpression>();
    if (creation == null) {
      _coverageMarker();
      return null;
    }
    ConstructorElement element = creation.staticElement;
    if (element == null ||
        element.name != 'fromIterable' ||
        element.enclosingElement != typeProvider.mapType.element) {
      _coverageMarker();
      return null;
    }
    //
    // Ensure that the arguments have the right form.
    //
    NodeList<Expression> arguments = creation.argumentList.arguments;
    if (arguments.length != 3) {
      _coverageMarker();
      return null;
    }
    Expression iterator = arguments[0].unParenthesized;
    Expression secondArg = arguments[1];
    Expression thirdArg = arguments[2];

    Expression extractBody(FunctionExpression expression) {
      FunctionBody body = expression.body;
      if (body is ExpressionFunctionBody) {
        return body.expression;
      } else if (body is BlockFunctionBody) {
        NodeList<Statement> statements = body.block.statements;
        if (statements.length == 1) {
          Statement statement = statements[0];
          if (statement is ReturnStatement) {
            return statement.expression;
          }
        }
      }
      return null;
    }

    FunctionExpression extractClosure(String name, Expression argument) {
      if (argument is NamedExpression && argument.name.label.name == name) {
        Expression expression = argument.expression.unParenthesized;
        if (expression is FunctionExpression) {
          NodeList<FormalParameter> parameters =
              expression.parameters.parameters;
          if (parameters.length == 1 && parameters[0].isRequiredPositional) {
            if (extractBody(expression) != null) {
              return expression;
            }
          }
        }
      }
      return null;
    }

    FunctionExpression keyClosure =
        extractClosure('key', secondArg) ?? extractClosure('key', thirdArg);
    FunctionExpression valueClosure =
        extractClosure('value', thirdArg) ?? extractClosure('value', secondArg);
    if (keyClosure == null || valueClosure == null) {
      _coverageMarker();
      return null;
    }
    //
    // Compute the loop variable name and convert the key and value closures if
    // necessary.
    //
    SimpleFormalParameter keyParameter = keyClosure.parameters.parameters[0];
    String keyParameterName = keyParameter.identifier.name;
    SimpleFormalParameter valueParameter =
        valueClosure.parameters.parameters[0];
    String valueParameterName = valueParameter.identifier.name;
    Expression keyBody = extractBody(keyClosure);
    String keyExpressionText = utils.getNodeText(keyBody);
    Expression valueBody = extractBody(valueClosure);
    String valueExpressionText = utils.getNodeText(valueBody);

    String loopVariableName;
    if (keyParameterName == valueParameterName) {
      loopVariableName = keyParameterName;
    } else {
      _ParameterReferenceFinder keyFinder =
          new _ParameterReferenceFinder(keyParameter.declaredElement);
      keyBody.accept(keyFinder);

      _ParameterReferenceFinder valueFinder =
          new _ParameterReferenceFinder(valueParameter.declaredElement);
      valueBody.accept(valueFinder);

      String computeUnusedVariableName() {
        String candidate = 'e';
        var index = 1;
        while (keyFinder.referencesName(candidate) ||
            valueFinder.referencesName(candidate)) {
          candidate = 'e${index++}';
        }
        return candidate;
      }

      if (valueFinder.isParameterUnreferenced) {
        if (valueFinder.referencesName(keyParameterName)) {
          // The name of the value parameter is not used, but we can't use the
          // name of the key parameter because doing so would hide a variable
          // referenced in the value expression.
          loopVariableName = computeUnusedVariableName();
          keyExpressionText = keyFinder.replaceName(
              keyExpressionText, loopVariableName, keyBody.offset);
        } else {
          loopVariableName = keyParameterName;
        }
      } else if (keyFinder.isParameterUnreferenced) {
        if (keyFinder.referencesName(valueParameterName)) {
          // The name of the key parameter is not used, but we can't use the
          // name of the value parameter because doing so would hide a variable
          // referenced in the key expression.
          loopVariableName = computeUnusedVariableName();
          valueExpressionText = valueFinder.replaceName(
              valueExpressionText, loopVariableName, valueBody.offset);
        } else {
          loopVariableName = valueParameterName;
        }
      } else {
        // The names are different and both are used. We need to find a name
        // that would not change the resolution of any other identifiers in
        // either the key or value expressions.
        loopVariableName = computeUnusedVariableName();
        keyExpressionText = keyFinder.replaceName(
            keyExpressionText, loopVariableName, keyBody.offset);
        valueExpressionText = valueFinder.replaceName(
            valueExpressionText, loopVariableName, valueBody.offset);
      }
    }
    //
    // Construct the edit.
    //
    DartChangeBuilder changeBuilder = _newDartChangeBuilder();
    await changeBuilder.addFileEdit(file, (DartFileEditBuilder builder) {
      builder.addReplacement(range.node(creation), (DartEditBuilder builder) {
        builder.write('{ for (var ');
        builder.write(loopVariableName);
        builder.write(' in ');
        builder.write(utils.getNodeText(iterator));
        builder.write(') ');
        builder.write(keyExpressionText);
        builder.write(' : ');
        builder.write(valueExpressionText);
        builder.write(' }');
      });
    });
    return changeBuilder;
  }

  Future<ChangeBuilder> createBuilder_convertToIntLiteral() async {
    if (node is! DoubleLiteral) {
      _coverageMarker();
      return null;
    }
    DoubleLiteral literal = node;
    int intValue;
    try {
      intValue = literal.value?.truncate();
    } catch (e) {
      // Double cannot be converted to int
    }
    if (intValue == null || intValue != literal.value) {
      _coverageMarker();
      return null;
    }

    var changeBuilder = _newDartChangeBuilder();
    await changeBuilder.addFileEdit(file, (DartFileEditBuilder builder) {
      builder.addReplacement(new SourceRange(literal.offset, literal.length),
          (DartEditBuilder builder) {
        builder.write('$intValue');
      });
    });
    return changeBuilder;
  }

  @protected
  Future<ChangeBuilder> createBuilder_useCurlyBraces() async {
    Future<ChangeBuilder> doStatement(DoStatement node) async {
      var body = node.body;
      if (body is Block) return null;

      var prefix = utils.getLinePrefix(node.offset);
      var indent = prefix + utils.getIndent(1);

      var changeBuilder = _newDartChangeBuilder();
      await changeBuilder.addFileEdit(file, (builder) {
        builder.addSimpleReplacement(
          range.endStart(node.doKeyword, body),
          ' {$eol$indent',
        );
        builder.addSimpleReplacement(
          range.endStart(body, node.whileKeyword),
          '$eol$prefix} ',
        );
      });
      return changeBuilder;
    }

    Future<ChangeBuilder> forStatement(ForStatement node) async {
      var body = node.body;
      if (body is Block) return null;

      var prefix = utils.getLinePrefix(node.offset);
      var indent = prefix + utils.getIndent(1);

      var changeBuilder = _newDartChangeBuilder();
      await changeBuilder.addFileEdit(file, (builder) {
        builder.addSimpleReplacement(
          range.endStart(node.rightParenthesis, body),
          ' {$eol$indent',
        );
        builder.addSimpleInsertion(body.end, '$eol$prefix}');
      });

      return changeBuilder;
    }

    Future<ChangeBuilder> ifStatement(
        IfStatement node, Statement thenOrElse) async {
      var prefix = utils.getLinePrefix(node.offset);
      var indent = prefix + utils.getIndent(1);

      var changeBuilder = _newDartChangeBuilder();
      await changeBuilder.addFileEdit(file, (builder) {
        var thenStatement = node.thenStatement;
        if (thenStatement is! Block &&
            (thenOrElse == null || thenOrElse == thenStatement)) {
          builder.addSimpleReplacement(
            range.endStart(node.rightParenthesis, thenStatement),
            ' {$eol$indent',
          );
          if (node.elseKeyword != null) {
            builder.addSimpleReplacement(
              range.endStart(thenStatement, node.elseKeyword),
              '$eol$prefix} ',
            );
          } else {
            builder.addSimpleInsertion(thenStatement.end, '$eol$prefix}');
          }
        }

        var elseStatement = node.elseStatement;
        if (elseStatement != null &&
            elseStatement is! Block &&
            (thenOrElse == null || thenOrElse == elseStatement)) {
          builder.addSimpleReplacement(
            range.endStart(node.elseKeyword, elseStatement),
            ' {$eol$indent',
          );
          builder.addSimpleInsertion(elseStatement.end, '$eol$prefix}');
        }
      });

      return changeBuilder;
    }

    Future<ChangeBuilder> whileStatement(WhileStatement node) async {
      var body = node.body;
      if (body is Block) return null;

      var prefix = utils.getLinePrefix(node.offset);
      var indent = prefix + utils.getIndent(1);

      var changeBuilder = _newDartChangeBuilder();
      await changeBuilder.addFileEdit(file, (builder) {
        builder.addSimpleReplacement(
          range.endStart(node.rightParenthesis, body),
          ' {$eol$indent',
        );
        builder.addSimpleInsertion(body.end, '$eol$prefix}');
      });
      return changeBuilder;
    }

    var statement = this.node.thisOrAncestorOfType<Statement>();
    var parent = statement?.parent;

    if (statement is DoStatement) {
      return doStatement(statement);
    } else if (parent is DoStatement) {
      return doStatement(parent);
    } else if (statement is ForStatement) {
      return forStatement(statement);
    } else if (parent is ForStatement) {
      return forStatement(parent);
    } else if (statement is IfStatement) {
      if (statement.elseKeyword != null &&
          range.token(statement.elseKeyword).contains(selectionOffset)) {
        return ifStatement(statement, statement.elseStatement);
      } else {
        return ifStatement(statement, null);
      }
    } else if (parent is IfStatement) {
      return ifStatement(parent, statement);
    } else if (statement is WhileStatement) {
      return whileStatement(statement);
    } else if (parent is WhileStatement) {
      return whileStatement(parent);
    }
    return null;
  }

  @protected
  bool setupCompute() {
    final locator = NodeLocator(selectionOffset, selectionEnd);
    node = locator.searchWithin(resolvedResult.unit);
    return node != null;
  }

  /// Configures [utils] using given [target].
  void _configureTargetLocation(Object target) {
    utils.targetClassElement = null;
    if (target is AstNode) {
      ClassDeclaration targetClassDeclaration =
          target.thisOrAncestorOfType<ClassDeclaration>();
      if (targetClassDeclaration != null) {
        utils.targetClassElement = targetClassDeclaration.declaredElement;
      }
    }
  }

  DartChangeBuilder _newDartChangeBuilder() =>
      DartChangeBuilderImpl.forWorkspace(workspace);

  /// This method does nothing, but we invoke it in places where Dart VM
  /// coverage agent fails to provide coverage information - such as almost
  /// all "return" statements.
  ///
  /// https://code.google.com/p/dart/issues/detail?id=19912
  static void _coverageMarker() {}
}

/// A visitor that can be used to find references to a parameter.
class _ParameterReferenceFinder extends RecursiveAstVisitor<void> {
  /// The parameter for which references are being sought, or `null` if we are
  /// just accumulating a list of referenced names.
  final ParameterElement parameter;

  /// A list of the simple identifiers that reference the [parameter].
  final List<SimpleIdentifier> references = <SimpleIdentifier>[];

  /// A collection of the names of other simple identifiers that were found. We
  /// need to know these in order to ensure that the selected loop variable does
  /// not hide a name from an enclosing scope that is already being referenced.
  final Set<String> otherNames = new Set<String>();

  /// Initialize a newly created finder to find references to the [parameter].
  _ParameterReferenceFinder(this.parameter) : assert(parameter != null);

  /// Return `true` if the parameter is unreferenced in the nodes that have been
  /// visited.
  bool get isParameterUnreferenced => references.isEmpty;

  /// Return `true` is the given name (assumed to be different than the name of
  /// the parameter) is references in the nodes that have been visited.
  bool referencesName(String name) => otherNames.contains(name);

  /// Replace all of the references to the parameter in the given [source] with
  /// the [newName]. The [offset] is the offset of the first character of the
  /// [source] relative to the start of the file.
  String replaceName(String source, String newName, int offset) {
    int oldLength = parameter.name.length;
    for (int i = references.length - 1; i >= 0; i--) {
      int oldOffset = references[i].offset - offset;
      source = source.replaceRange(oldOffset, oldOffset + oldLength, newName);
    }
    return source;
  }

  @override
  void visitSimpleIdentifier(SimpleIdentifier node) {
    if (node.staticElement == parameter) {
      references.add(node);
    } else if (!node.isQualified) {
      // Only non-prefixed identifiers can be hidden.
      otherNames.add(node.name);
    }
  }
}