Fixed custom elements to work in Polymer

tools/dom/templates/html/dart2js/html_dart2js.darttemplate

 // Copyright (c) 2012, 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.
 
 // DO NOT EDIT - unless you are editing documentation as per:
 // https://code.google.com/p/dart/wiki/ContributingHTMLDocumentation
 // Auto-generated dart:html library.
 
 /**
  * HTML elements and other resources for web-based applications that need to
@@ skipping 131 matching lines @@
  * Spawn a DOM isolate using the given URI in the same window.
  * This isolate is not concurrent.  It runs on the browser thread
  * with full access to the DOM.
  * Note: this API is still evolving and may move to dart:isolate.
  */
 @Experimental()
 Future<Isolate> spawnDomUri(Uri uri, List<String> args, message) {
   // TODO(17738): Implement this.
   throw new UnimplementedError();
 }
+
+/// Dartium functions that are a NOOP in dart2js.
+unwrap_jso(dartClass_instance) => dartClass_instance;
+wrap_jso(jsObject) => jsObject;
+createCustomUpgrader(Type customElementClass, $this) => $this;