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:     if (supertype == null) {
        Supertype targetSupertype = cls.supertype;
        if (targetSupertype == null) return null;
        DillLoader loader = library.loader;
        super.supertype = supertype =
            loader.computeTypeBuilder(targetSupertype.asInterfaceType);
      }
      return supertype;
    }
  
negative.code:   /// superclass.
    bool get isMixinApplication => cls.isMixinApplication;
  
    KernelTypeBuilder get mixedInType {
      return computeTypeBuilder(library, cls.mixedInType);
    }
  
    void set mixedInType(KernelTypeBuilder mixin) {
      unimplemented("mixedInType=", -1, null);
  