Update to use one less proxy per element

lib/src/common/polymer_mixin.dart

 // Copyright (c) 2015, 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.src.common.polymer_js_proxy;
 
 import 'dart:js';
 import 'package:polymer_interop/polymer_interop.dart' show Polymer, PolymerDom;
 import 'package:web_components/web_components.dart';
 import 'js_proxy.dart';
 
 /// Basic api for re-using the polymer js prototypes.
 abstract class PolymerMixin implements JsProxy, CustomElementProxyMixin {
   JsObject _proxy;
 
   JsObject get jsElement {
     if (_proxy == null) {
       _proxy = new JsObject.fromBrowserObject(this);
-      _proxy['__data__'] = jsProxy;
     }
     return _proxy;
   }
 
   void polymerCreated() {
-    // Use a cache for js proxy values.
-    useCache = true;
     jsElement.callMethod('originalPolymerCreatedCallback');
   }
+
+
 }