Update to use one less proxy per element

lib/src/common/js_proxy.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.lib.src.common.js_proxy;
 
 import 'dart:js';
 import 'package:polymer_interop/polymer_interop.dart';
 export 'package:polymer_interop/polymer_interop.dart' show dartValue, jsValue;
 import 'package:reflectable/reflectable.dart';
 import 'behavior.dart';
@@ skipping 44 matching lines @@
       : super(
             instanceInvokeCapability,
             metadataCapability,
             declarationsCapability,
             typeCapability,
             typeRelationsCapability,
             subtypeQuantifyCapability,
             superclassQuantifyCapability,
             const StaticInvokeCapability('hostAttributes'));
 }
-
 const jsProxyReflectable = const JsProxyReflectable();
 
 final JsObject _polymerDart = context['Polymer']['Dart'];
 
 /// Given a dart type, this creates a javascript constructor and prototype
 /// which can act as a proxy for it.
 JsFunction _buildJsConstructorForType(Type dartType) {
   var constructor = _polymerDart.callMethod('functionFactory');
   var prototype = new JsObject(context['Object']);
 
@@ skipping 35 matching lines @@
           var mirror = jsProxyReflectable.reflect(dartInstance);
           return jsValue(mirror.invoke(name, newArgs));
         }
       ]);
     }
   });
 
   constructor['prototype'] = prototype;
   return constructor;
 }