anchor.diff:  int computeModifiers(Class cls) {
     return cls.isAbstract ? abstractMask : 0;
   }
  +
  +KernelTypeBuilder computeTypeBuilder(
  +    DillLibraryBuilder library, Supertype supertype) {
  +  return supertype == null
  +      ? null
  +      : library.loader.computeTypeBuilder(supertype.asInterfaceType);
  +}
  
positive.code: import '../modifier.dart' show abstractMask;
  
  import 'dill_library_builder.dart' show DillLibraryBuilder;
  
  import 'dill_loader.dart' show DillLoader;
  
  import 'dill_member_builder.dart' show DillMemberBuilder;
  
  
negative.code:     if (supertype == null) {
        Supertype targetSupertype = cls.supertype;
        if (targetSupertype == null) return null;
        super.supertype =
            supertype = computeTypeBuilder(library, targetSupertype);
      }
      return supertype;
    }
  