anchor.diff:    factory ExternalClassBuilder() = _$ExternalClassBuilder;
   }
   
  -abstract class Package implements NamedElement, Built<Package, PackageBuilder> {
  +abstract class Package implements Built<Package, PackageBuilder>, NamedElement {
     static final Serializer<Package> serializer = _$packageSerializer;
     String get name;
     BuiltSet<Classifier> get classifiers;
  
positive.code:   factory PropertyBuilder() = _$PropertyBuilder;
  }
  
  abstract class ValueClass extends GenericClassifier
      implements Built<ValueClass, ValueClassBuilder> {
    static final Serializer<ValueClass> serializer = _$valueClassSerializer;
    String get name;
    BuiltSet<TypeParameter> get genericTypes;
  
negative.code: }
  
  abstract class GenericType
      implements Built<GenericType, GenericTypeBuilder>, Classifier {
    static final Serializer<GenericType> serializer = _$genericTypeSerializer;
    String get name;
    Classifier get base;
  