Type extension support in polyfill.

tools/dom/src/dart2js_CustomElementSupport.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.
 
 part of dart.dom.html;
 
 _callCreated(receiver) {
   return receiver.created();
 }
 
 _makeCreatedCallbackMethod() {
   return JS('',
       '''((function(invokeCallback) {
              return function() {
                return invokeCallback(this);
              };
           })(#))''',
       convertDartClosureToJS(_callCreated, 1));
 }
 
-void _registerCustomElement(context, document, String tag, Type type) {
+const _typeNameToTag = const {
+  'HTMLAnchorElement': 'a',
+  'HTMLAudioElement': 'audio',
+  'HTMLButtonElement': 'button',
+  'HTMLCanvasElement': 'canvas',
+  'HTMLDivElement': 'div',
+  'HTMLImageElement': 'img',
+  'HTMLInputElement': 'input',
+  'HTMLLIElement': 'li',
+  'HTMLLabelElement': 'label',
+  'HTMLMenuElement': 'menu',
+  'HTMLMeterElement': 'meter',
+  'HTMLOListElement': 'ol',
+  'HTMLOptionElement': 'option',
+  'HTMLOutputElement': 'output',
+  'HTMLParagraphElement': 'p',
+  'HTMLPreElement': 'pre',
+  'HTMLProgressElement': 'progress',
+  'HTMLSelectElement': 'select',
+  'HTMLSpanElement': 'span',
+  'HTMLUListElement': 'ul',
+  'HTMLVideoElement': 'video',
+};
+
+void _registerCustomElement(context, document, String tag, Type type,
+    String nativeTagName) {
   // Function follows the same pattern as the following JavaScript code for
   // registering a custom element.
   //
   //    var proto = Object.create(HTMLElement.prototype, {
   //        createdCallback: {
   //          value: function() {
   //            window.console.log('here');
   //          }
   //        }
   //    });
@@ skipping 21 matching lines @@
   JS('void', '#.createdCallback = #', properties,
       JS('=Object', '{value: #}', jsCreatedCallback));
 
   var baseProto = JS('=Object', '#.prototype', baseConstructor);
   var proto = JS('=Object', 'Object.create(#, #)', baseProto, properties);
 
   var interceptor = JS('=Object', '#.prototype', interceptorClass);
 
   setNativeSubclassDispatchRecord(proto, interceptor);
 
-  JS('void', '#.register(#, #)',
-      document, tag, JS('', '{prototype: #}', proto));
+  var options = JS('=Object', '{prototype: #}', proto);
+
+  if (baseClassName != 'HTMLElement') {
+    if (nativeTagName != null) {
+      JS('=Object', '#.extends = #', options, nativeTagName);
+    } else if (_typeNameToTag.containsKey(baseClassName)) {
+      JS('=Object', '#.extends = #', options, _typeNameToTag[baseClassName]);
+    }
+  }
+
+  JS('void', '#.register(#, #)', document, tag, options);
 }