move workaround from CE polyfill to custom elements createdCallback

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);
 
+    // Mirrors uses the constructor property to cache lookups, so we need it to
+    // be set correctly, including on IE where it is not automatically picked
+    // up from the __proto__.
+    JS('', '#.constructor = #.__proto__.constructor', receiver, receiver);
     return JS('', '#(#)', constructor, receiver);
   };
 }
 
 _callEnteredView(receiver) {
   return receiver.enteredView();
 }
 
 _callLeftView(receiver) {
   return receiver.leftView();
@@ skipping 96 matching lines @@
     JS('=Object', '#.extends = #', options, extendsTagName);
   }
 
   JS('void', '#.registerElement(#, #)', 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.
 }