Update to use one less proxy per element

lib/src/common/polymer_mixin.dart

Index: lib/src/common/polymer_mixin.dart
diff --git a/lib/src/common/polymer_mixin.dart b/lib/src/common/polymer_mixin.dart
index 5b69770261293ea1817406cd2f423cecc88edfbf..c85db34d1d89f2df546137ad4995d6ab2efce60e 100644
--- a/lib/src/common/polymer_mixin.dart
+++ b/lib/src/common/polymer_mixin.dart
@@ -15,14 +15,13 @@ abstract class PolymerMixin implements JsProxy, CustomElementProxyMixin {
   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');
   }
+
+
 }