Fixed warning causing drone.io to fail

lib/polymer_element.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.
 
 library polymer.polymer_element;
 
 import 'dart:async';
 import 'dart:html';
 import 'dart:mirrors';
 
@@ skipping 156 matching lines @@
     if (propObserver != null) {
       return new _PolymerBinding(this, name, model, path, propObserver);
     }
     return super.createBinding(name, model, path);
   }
 
   /**
    * Using Polymer's platform/src/ShadowCSS.js passing the style tag's content.
    */
   void _shimCss(ShadowRoot root, String localName, String extendsName) {
-    var platform = js.context.Platform;
+    // TODO(terry): Remove warning, cast js.context to dynamic because of bug
+    //              https://code.google.com/p/dart/issues/detail?id=6111. The
+    //              js interop package will be patching this until bug is fixed.
+    var platform = (js.context as dynamic).Platform;
     if (platform == null) return;
     var shadowCss = platform.ShadowCSS;
     if (shadowCss == null) return;
 
     // TODO(terry): Remove calls to shimShadowDOMStyling2 and replace with
     //              shimShadowDOMStyling when we support unwrapping dart:html
     //              Element to a JS DOM node.
     var shimShadowDOMStyling2 = shadowCss.shimShadowDOMStyling2;
     if (shimShadowDOMStyling2 == null) return;
     var style = root.query('style');
@@ skipping 355 matching lines @@
   'onKeyMessage': MediaElement.keyMessageEvent,
   'onNeedKey': MediaElement.needKeyEvent,
   'onWebGlContextLost': CanvasElement.webGlContextLostEvent,
   'onWebGlContextRestored': CanvasElement.webGlContextRestoredEvent,
   'onPointerLockChange': Document.pointerLockChangeEvent,
   'onPointerLockError': Document.pointerLockErrorEvent,
   'onReadyStateChange': Document.readyStateChangeEvent,
   'onSelectionChange': Document.selectionChangeEvent,
   'onSecurityPolicyViolation': Document.securityPolicyViolationEvent,
 };