anchor.diff:    return new ValueClass((cb) => cb
       ..name = 'Classifier'
       ..isAbstract = true
  -    ..superTypes.add(namedElement));
  +    ..superTypes.add(namedElement.toBuilder()));
   }
   
   ValueClass _typeParameter;
  
positive.code: 
  ValueClass _createExternalClass() => new ValueClass((cb) => cb
    ..name = 'ExternalClass'
    ..superTypes.add(genericClassifier));
  
  ValueClass _enumClass;
  ValueClass get enumClass => _enumClass ??= _createEnumClass();
  
negative.code:   return new ValueClass((cb) => cb
      ..name = 'NamedElement'
      ..isAbstract = true
      ..superTypes.add(modelElement.toBuilder())
      ..properties.add(new PropertyBuilder()
        ..name = 'name'
        ..type = dartString.toBuilder()
  