anchor.diff:  
     List<Procedure> methods = <Procedure>[];
   
  -  factory ClassDeclaration(Reference classRef) {
  -    if (_classes.containsKey(classRef)) {
  -      return _classes[classRef];
  -    }
  -    _classes[classRef] = new ClassDeclaration._internal(classRef.asClass);
  -    return _classes[classRef];
  -  }
  -
  -  ClassDeclaration._internal(Class currentClass) {
  +  int get instanceSize => instanceFields.length;
  +
  +  factory Class(Reference classRef) {
  +    return _classes.putIfAbsent(
  +        classRef, () => new Class._internal(classRef.asClass));
  +  }
  +
  +  Class._internal(ast.Class currentClass) {
       if (currentClass.superclass != null) {
  -      superclass = new ClassDeclaration(currentClass.superclass.reference);
  +      superclass = new Class(currentClass.superclass.reference);
       }
   
       _populateInstanceFields(currentClass);
  -
  -    // Populate implicit setters and getters.
  -    for (int i = 0; i < instanceFields.length; i++) {
  -      Field f = instanceFields[i];
  -      assert(f.hasImplicitGetter);
  -      getters[f.name] = (Value receiver) => receiver.fields[i];
  -      if (f.hasImplicitSetter) {
  -        setters[f.name] =
  -            (Value receiver, Value value) => receiver.fields[i] = value;
  -      }
  -    }
       // TODO: Populate methods.
     }
   
  
positive.code:     return notImplemented(obj: member);
    }
  
    // Populates with the instance fields of the current class and all its
    // superclasses recursively.
    _populateInstanceFields(Class class_) {
      for (Field f in class_.fields) {
        if (f.isInstanceMember) {
          instanceFields.add(f);
        }
      }
  
      if (class_.superclass != null) {
        _populateInstanceFields(class_.superclass);
      }
    }
  }
  
  abstract class Value {
    ClassDeclaration get classDeclaration;
    List<Value> get fields;
    Object get value;
  
  
negative.code: library kernel.interpreter;
  
  import '../ast.dart';
  import '../ast.dart' as ast show Class;
  
  class NotImplemented {
    String message;
  