regenerate fixed custom_elements polyfill

pkg/custom_element/lib/polyfill.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.
 
 /** Dart APIs for interacting with the JavaScript Custom Elements polyfill. */
 library custom_element.polyfill;
 
 import 'dart:async';
 import 'dart:html';
 import 'dart:js' as js;
@@ skipping 36 matching lines @@
  * Loads `custom-elements.debug.js` or `custom-elements.min.js` by adding the
  * script tag to the page. Returns a future that completes when custom elements
  * are ready (equivalent to [customElementsReady]).
  *
  * Normally you should add this to your HTML file
  * (the Polymer package will do this automatically), but loading dynamically
  * can be useful for scenarios such as tests.
  */
 Future loadCustomElementPolyfill() {
   if (!document.supportsRegister && !js.context.hasProperty('CustomElements')) {
-    if (!MutationObserver.supported) {
-      var script = new ScriptElement()
-          ..src = '/packages/mutation_observer/mutation_observer.js';
-      document.head.append(script);
-    }
-
     var script = new ScriptElement()
         ..src = '/packages/custom_element/custom-elements.debug.js';
     document.head.append(script);
     return document.on['WebComponentsReady'].first;
   }
   return new Future.value();
 }