[polymer] fix loader on IE

pkg/polymer/lib/src/loader.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 polymer;
 
 /// Annotation used to automatically register polymer elements.
 class CustomTag {
   final String tagName;
   const CustomTag(this.tagName);
@@ skipping 63 matching lines @@
         ' the platform polyfills.');
   }
 
   // TODO(jmesserly): dart:js appears to not callback in the correct zone:
   // https://code.google.com/p/dart/issues/detail?id=17301
   var zone = Zone.current;
 
   polymerJs.callMethod('whenPolymerReady',
       [zone.bindCallback(() => Polymer._ready.complete())]);
 
-  var jsPolymer = new JsObject.fromBrowserObject(
-      document.createElement('polymer-element'));
-
-  var proto = js.context['Object'].callMethod('getPrototypeOf', [jsPolymer]);
-  if (proto is Node) {
-    proto = new JsObject.fromBrowserObject(proto);
-  }
+  var polyElem = document.createElement('polymer-element');
+  var proto = new JsObject.fromBrowserObject(polyElem)['__proto__'];
+  if (proto is Node) proto = new JsObject.fromBrowserObject(proto);
 
   JsFunction originalRegister = proto['register'];
   if (originalRegister == null) {
     throw new StateError('polymer.js must expose "register" function on '
         'polymer-element to enable polymer.dart to interoperate.');
   }
 
   registerDart(jsElem, String name, String extendee) {
     // By the time we get here, we'll know for sure if it is a Dart object
     // or not, because polymer-element will wait for us to notify that
     // the @CustomTag was found.
     final type = _getRegisteredType(name);
     if (type != null) {
       final extendsDecl = _getDeclaration(extendee);
       return zone.run(() =>
           new PolymerDeclaration(jsElem, name, type, extendsDecl).register());
     }
     // It's a JavaScript polymer element, fall back to the original register.
     return originalRegister.apply([name, extendee], thisArg: jsElem);
   }
 
   proto['register'] = new JsFunction.withThis(registerDart);
 }