move workaround from CE polyfill to custom elements createdCallback

pkg/custom_element/lib/custom-elements.debug.js

 // Copyright (c) 2012 The Polymer Authors. All rights reserved.
 //
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //    * Redistributions of source code must retain the above copyright
 // notice, this list of conditions and the following disclaimer.
 //    * Redistributions in binary form must reproduce the above
 // copyright notice, this list of conditions and the following disclaimer
@@ skipping 944 matching lines @@
   scope = window.CustomElements = {flags:{}};
 }
 var flags = scope.flags;
 
 // native document.registerElement?
 
 var hasNative = Boolean(document.registerElement);
 // TODO(sorvell): See https://github.com/Polymer/polymer/issues/399
 // we'll address this by defaulting to CE polyfill in the presence of the SD
 // polyfill. This will avoid spamming excess attached/detached callbacks.
-// If there is a compelling need to run CE native with SD polyfill, 
+// If there is a compelling need to run CE native with SD polyfill,
 // we'll need to fix this issue.
 var useNative = !flags.register && hasNative && !window.ShadowDOMPolyfill;
 
 if (useNative) {
 
   // stub
   var nop = function() {};
 
   // exports
   scope.registry = {};
@@ skipping 192 matching lines @@
 
   function implement(element, definition) {
     // prototype swizzling is best
     if (Object.__proto__) {
       element.__proto__ = definition.prototype;
     } else {
       // where above we can re-acquire inPrototype via
       // getPrototypeOf(Element), we cannot do so when
       // we use mixin, so we install a magic reference
       customMixin(element, definition.prototype, definition.native);
-
-      // Dart note: make sure we pick up the right constructor.
-      // dart2js depends on this for dart:mirrors caching to work.
-      // See tests/html/custom/mirrors_test.dart
-      element.constructor = definition.prototype.constructor;
       element.__proto__ = definition.prototype;
     }
   }
 
   function customMixin(inTarget, inSrc, inNative) {
     // TODO(sjmiles): 'used' allows us to only copy the 'youngest' version of
     // any property. This set should be precalculated. We also need to
     // consider this for supporting 'super'.
     var used = {};
     // start with inSrc
@@ skipping 324 matching lines @@
 if (window.CustomElements && !CustomElements.useNative) {
   var originalImportNode = Document.prototype.importNode;
   Document.prototype.importNode = function(node, deep) {
     var imported = originalImportNode.call(this, node, deep);
     CustomElements.upgradeAll(imported);
     return imported;
   }
 }
 
 })();