fix custom elements that extend tags like "section"

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;
 
 _callConstructor(constructor, interceptor) {
   return (receiver) {
     setNativeSubclassDispatchRecord(receiver, interceptor);
 
@@ skipping 95 matching lines @@
   JS('void', '#.attributeChangedCallback = #', properties,
       JS('=Object', '{value: #}', _makeCallbackMethod3(_callAttributeChanged)));
 
   var baseProto = JS('=Object', '#.prototype', baseConstructor);
   var proto = JS('=Object', 'Object.create(#, #)', baseProto, properties);
 
   setNativeSubclassDispatchRecord(proto, interceptor);
 
   var options = JS('=Object', '{prototype: #}', proto);
 
-  if (baseClassName != 'HTMLElement') {
-    if (extendsTagName != null) {
-      JS('=Object', '#.extends = #', options, extendsTagName);
-    }
+  if (extendsTagName != null) {
+    JS('=Object', '#.extends = #', options, extendsTagName);
   }
 
   JS('void', '#.register(#, #)', document, tag, options);
 }
 
 //// Called by Element.created to do validation & initialization.
 void _initializeCustomElement(Element e) {
   // TODO(blois): Add validation that this is only in response to an upgrade.
 }