pkg/mdv: remove reference to pkg/serialization

pkg/mdv/lib/mdv.dart

 // Copyright (c) 2013, 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.
 
 // TODO(jmesserly): more commentary here.
 /**
  * This library provides access to Model-Driven-Views APIs on HTML elements.
  * More information can be found at: <https://github.com/Polymer/mdv>.
  */
 library mdv;
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:html';
 import 'package:observe/observe.dart';
 
-// TODO(jmesserly): get this from somewhere else. See http://dartbug.com/4161.
-import 'package:serialization/src/serialization_helpers.dart' show IdentityMap;
-
 import 'src/list_diff.dart' show calculateSplices, ListChangeDelta;
 
 part 'src/element.dart';
 part 'src/input_bindings.dart';
 part 'src/input_element.dart';
 part 'src/node.dart';
 part 'src/select_element.dart';
 part 'src/template.dart';
 part 'src/template_iterator.dart';
 part 'src/text.dart';
@@ skipping 63 matching lines @@
   } else if (node is Node) {
     wrapper = new _NodeExtension(node);
   } else {
     // TODO(jmesserly): this happens for things like CompoundBinding.
     wrapper = node;
   }
 
   _mdvExpando[node] = wrapper;
   return wrapper;
 }