anchor.diff:  
   import 'dart:collection';
   
  -import 'package:analyzer/dart/ast/ast.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/src/dart/element/element.dart';
  
positive.code:     _FieldOverrideInferenceResult typeResult =
          _computeFieldOverrideType(field.getter);
      if (typeResult.isError) {
        if (field is FieldElementImpl && field.linkedNode != null) {
          LazyAst.setTypeInferenceError(
            field.linkedNode,
            TopLevelInferenceErrorBuilder(
  
negative.code: }
  
  /**
   * A class of exception that is not used anywhere else.
   */
  class _CycleException implements Exception {}
  