Support for extending native classes

tests/html/custom_elements_test.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library custom_elements_test;
 import '../../pkg/unittest/lib/unittest.dart';
 import '../../pkg/unittest/lib/html_individual_config.dart';
 import 'dart:html';
 
 class CustomType extends Element {
   factory CustomType() => null;
-
   bool onCreatedCalled = false;
   void onCreated() {
     onCreatedCalled = true;
     customCreatedCount++;
   }
 }
 
 int customCreatedCount = 0;
 
 class NotAnElement {}
@@ skipping 97 matching lines @@
       int oldCount = customCreatedCount;
 
       document.register('x-type9', CustomType);
       var element = new DivElement();
       element.innerHtml = '<x-type9></x-type9>';
       document.body.nodes.add(element);
       expect(customCreatedCount, oldCount + 1);
     });
   });
 }