anchor.diff:        if (isDom || isJsInterop) {
           // TODO(jacobr): verify class implements JavaScriptObject.
           var className = mirrors.MirrorSystem.getName(clazz.simpleName);
  -        bool isPrivate = className.startsWith('_');
  +        bool isPrivateUserDefinedClass =
  +            className.startsWith('_') && !dartLibrary;
           var classNameImpl = '${className}Impl';
           var sbPatch = new StringBuffer();
           if (isJsInterop) {
  
positive.code: 
    for (var typeMirror in jsInterfaceTypes) {
      mirrors.LibraryMirror libraryMirror = typeMirror.owner;
      var prefixName;
      if (libraryPrefixes.containsKey(libraryMirror)) {
        prefixName = libraryPrefixes[libraryMirror];
  
negative.code:           sbPatch.write(
                "  static Type get instanceRuntimeType => ${classNameImpl};\n");
          }
          if (isPrivateUserDefinedClass) {
            sb.write("""
  class ${escapePrivateClassPrefix}${className} implements $className {}
  """);
  