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;
   }
 }
 
 class NotAnElement {}
 
 main() {
   useHtmlIndividualConfiguration();
@@ skipping 68 matching lines @@
       postElement.onFocus.listen((_) {
         firedOnPost = true;
       });
       // Event handlers should not persist to new element.
       postElement.dispatchEvent(new Event('focus'));
       expect(firedOnPre, isFalse);
       expect(firedOnPost, isTrue);
     });
   });
 }