anchor.diff:   * during linking.
    */
   class ConstructorElementForLink extends ExecutableElementForLink
  -    implements ConstructorElementImpl, ReferenceableElementForLink {
  +    with ReferenceableElementForLink
  +    implements ConstructorElementImpl {
     /**
      * If this is a `const` constructor and the enclosing library is
      * part of the build unit being linked, the constructor's node in
  
positive.code:       : super(enclosingUnit, null, _buf);
  
    @override
    ConstructorElementForLink get asConstructor => null;
  
    @override
    ConstVariableNode get asConstVariable => null;
  
    @override
    DartType get asStaticType => type;
  
    @override
    TypeInferenceNode get asTypeInferenceNode => null;
  
    @override
    ElementKind get kind => ElementKind.FUNCTION;
  
    @override
    DartType buildType(DartType getTypeArgument(int i),
            List<int> implicitFunctionTypeIndices) =>
        DynamicTypeImpl.instance;
  
    @override
    ReferenceableElementForLink getContainedName(String name) {
      // TODO(paulberry): handle references to `call`.
      return UndefinedElementForLink.instance;
    }
  
    @override
    noSuchMethod(Invocation invocation) => super.noSuchMethod(invocation);
  
negative.code: /**
   * Singleton element used for unresolved references.
   */
  class UndefinedElementForLink extends Object with ReferenceableElementForLink {
    static final UndefinedElementForLink instance =
        new UndefinedElementForLink._();
  
    UndefinedElementForLink._();
  }
  
  /**
  