anchor.diff:    /// superclass.
     bool get isMixinApplication => cls.isMixinApplication;
   
  -  KernelTypeBuilder get mixedInType => unimplemented("mixedInType", -1, null);
  +  KernelTypeBuilder get mixedInType {
  +    return computeTypeBuilder(library, cls.mixedInType);
  +  }
   
     void set mixedInType(KernelTypeBuilder mixin) {
       unimplemented("mixedInType=", -1, null);
  
positive.code: int computeModifiers(Class cls) {
    return cls.isAbstract ? abstractMask : 0;
  }
  
negative.code:     if (supertype == null) {
        Supertype targetSupertype = cls.supertype;
        if (targetSupertype == null) return null;
        super.supertype =
            supertype = computeTypeBuilder(library, targetSupertype);
      }
      return supertype;
    }
  