anchor.diff:  }
   
   abstract class PackageBuilder implements Builder<Package, PackageBuilder> {
  +  @nullable
  +  String docComment;
     String name;
     SetBuilder<Classifier> classifiers = new SetBuilder<Classifier>();
   
  
positive.code: 
  abstract class ExternalClassBuilder
      implements Builder<ExternalClass, ExternalClassBuilder> {
    String name;
    SetBuilder<TypeParameter> genericTypes = new SetBuilder<TypeParameter>();
  
  
negative.code:     implements Built<ExternalClass, ExternalClassBuilder>, GenericClassifier {
    static final Serializer<ExternalClass> serializer = _$externalClassSerializer;
  
    @nullable
    String get docComment;
    String get name;
    BuiltSet<TypeParameter> get genericTypes;
  
  