Update to use one less proxy per element

lib/src/js/polymer_bind_dart.html

 <!--
 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.
 -->
 <script>
 if (Polymer.Bind) {
-  // Overwrite prepareInstance to a no-op for dart elements.
-  Polymer.Bind._originalPrepareInstance = Polymer.Bind.prepareInstance;
-  Polymer.Bind.prepareInstance = function (inst) {
-    if (inst.__isPolymerDart__) return;
-    this._originalPrepareInstance(inst);
-  };
-
   // We always need to create property accessors for dart elements, because we
   // always have `effects` (updating the dart class).
   // TODO(jakemac): Investigate making the updating of the dart class be an
   // actual `effect` so that we don't have to do this hack.
   Polymer.Bind.oldCreateBindings = Polymer.Bind.createBindings;
   Polymer.Bind.createBindings = function(model) {
     if (!model.__isPolymerDart__) {
       this.oldCreateBindings(model);
       return;
     }
@@ skipping 20 matching lines @@
       // effects have priority
       if (fx) {
         fx.sort(this._sortPropertyEffects);
       }
       // create accessors
       this._createAccessors(model, n, fx);
     }
   };
 }
 </script>