anchor.diff:  import 'package:analyzer/dart/ast/ast.dart';
   import 'package:analyzer/dart/element/element.dart';
   import 'package:analyzer/dart/element/type.dart';
  -
  +import 'package:analyzer/src/generated/source.dart';
   import 'package:front_end/src/fasta/builder/builder.dart' show Builder;
  -
   import 'package:front_end/src/fasta/errors.dart' show internalError;
  -import 'package:analyzer/src/generated/source.dart';
   
   abstract class MockElement extends Builder implements Element, LocalElement {
  +  @override
     final ElementKind kind;
   
     MockElement(this.kind) : super(null, -1, null);
   
  +  @override
     get librarySource => internalError("not supported.");
   
  +  @override
     get source => internalError("not supported.");
   
  +  @override
     get context => internalError("not supported.");
   
  +  @override
     String get displayName => internalError("not supported.");
   
  +  @override
     String get documentationComment => internalError("not supported.");
   
  +  @override
     Element get enclosingElement => internalError("not supported.");
   
  +  @override
     int get id => internalError("not supported.");
   
  +  @override
     bool get isDeprecated => internalError("not supported.");
   
  +  @override
     bool get isFactory => internalError("not supported.");
   
  +  @override
     bool get isJS => internalError("not supported.");
   
  +  @override
     bool get isOverride => internalError("not supported.");
   
  +  @override
     bool get isPrivate => internalError("not supported.");
   
  +  @override
     bool get isProtected => internalError("not supported.");
   
  +  @override
     bool get isPublic => internalError("not supported.");
   
  +  @override
     bool get isRequired => internalError("not supported.");
   
  +  @override
     bool get isSynthetic => internalError("not supported.");
   
  +  @override
     LibraryElement get library => internalError("not supported.");
   
  +  @override
     get location => internalError("not supported.");
   
  +  @override
     get metadata => internalError("not supported.");
   
  +  @override
     String get name => internalError("not supported.");
   
  +  @override
     String get fullNameForErrors => name;
   
  +  @override
     int get nameLength => internalError("not supported.");
   
  +  @override
     int get nameOffset => -1;
   
  +  @override
     get unit => internalError("not supported.");
   
  +  @override
     accept<T>(visitor) => internalError("not supported.");
   
  +  @override
     String computeDocumentationComment() => internalError("not supported.");
   
  +  @override
     computeNode() => internalError("not supported.");
   
  +  @override
     getAncestor<E>(predicate) => internalError("not supported.");
   
  +  @override
     String getExtendedDisplayName(String shortName) {
       return internalError("not supported.");
     }
   
  +  @override
     bool isAccessibleIn(LibraryElement library) {
       return internalError("not supported.");
     }
   
  +  @override
     void visitChildren(visitor) => internalError("not supported.");
   
     String get uri => internalError("not supported.");
  
positive.code:     implements CompilationUnitElement {
    MockCompilationUnitElement() : super(ElementKind.COMPILATION_UNIT);
  
    List<PropertyAccessorElement> get accessors {
      return internalError("not supported.");
    }
  
    LibraryElement get enclosingElement => internalError("not supported.");
  
    List<ClassElement> get enums => internalError("not supported.");
  
    List<FunctionElement> get functions => internalError("not supported.");
  
    List<FunctionTypeAliasElement> get functionTypeAliases {
      return internalError("not supported.");
    }
  
    bool get hasLoadLibraryFunction => internalError("not supported.");
  
    @override
    LineInfo get lineInfo => internalError("not supported.");
  
    List<TopLevelVariableElement> get topLevelVariables {
      return internalError("not supported.");
    }
  
    List<ClassElement> get types => internalError("not supported.");
  
    ClassElement getEnum(String name) => internalError("not supported.");
  
    ClassElement getType(String name) => internalError("not supported.");
  
    CompilationUnit computeNode() => internalError("not supported.");
  }
  
  
negative.code: 
    bool get isOperator => internalError("not supported.");
  
    @override
    bool get isStatic => internalError("not supported.");
  
    bool get isSynchronous => internalError("not supported.");
  
    List<LabelElement> get labels => internalError("not supported.");
  
    @override
    get visibleRange => internalError("not supported.");
  
    bool get hasImplicitType => internalError("not supported.");
  
    FunctionElement get initializer => internalError("not supported.");
  
    @override
    bool get isConst => internalError("not supported.");
  
    @override
    bool get isFinal => internalError("not supported.");
  
    bool get isPotentiallyMutatedInClosure => internalError("not supported.");
  