Update to use one less proxy per element

lib/src/common/polymer_serialize.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_serialize;
 
 import 'dart:js';
 import 'js_proxy.dart';
 import 'polymer_mixin.dart';
 import 'polymer_descriptor.dart';
 
 /// Mixin for Polymer serialization methods.
 ///
 /// This should only be used if the [serialize] and [deserialize] methods need
 /// to be overridden to support additional Dart types. Any types not explicitly
 /// handled by the overridden method should defer to the original method by
 /// calling the base class's implementation.
 abstract class PolymerSerialize implements PolymerMixin {
   JsObject get jsElement;
 
   /// Serializes the [value] into a [String].
-  String serialize(Object value) {
-    var result = jsElement.callMethod('originalSerialize', [jsValue(value)]);
+  String serialize(value) {
+    var result = _polymerDartSerialize.apply([jsValue(value)]);
 
     return (result != null) ? result.toString() : null;
   }
 
   /// Deserializes the [value] into an object of the given [type].
   dynamic deserialize(String value, Type type) {
-    return dartValue(jsElement.callMethod(
-        'originalDeserialize', [jsValue(value), jsType(type)]));
+    return dartValue(_polymerDartDeserialize.apply([value, jsType(type)]));
   }
 }
+
+final JsObject _polymer = context['Polymer'];
+final JsFunction _polymerDartSerialize = _polymer['Dart']['serialize'];
+final JsFunction _polymerDartDeserialize = _polymer['Dart']['deserialize'];