Get rid of the polyfill identity set implementation in favour of a

pkg/serialization/lib/src/reader_writer.dart

 // Copyright (c) 2012, 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.
 
 part of serialization;
 
 /**
  * This writes out the state of the objects to an external format. It holds
  * all of the intermediate state needed. The primary API for it is the
  * [write] method.
@@ skipping 20 matching lines @@
    */
   bool selfDescribing;
 
   /**
    * Objects that cannot be represented in-place in the serialized form need
    * to have references to them stored. The [Reference] objects are computed
    * once and stored here for each object. This provides some space-saving,
    * but also serves to record which objects we have already seen.
    */
   final Map<Object, Reference> references =
-      new IdentityMapPlus<Object, Reference>();
+      new IdentityMap<Object, Reference>();
 
   /**
    * The state of objects that need to be serialized is stored here.
    * Each rule has a number, and rules keep track of the objects that they
    * serialize, in order. So the state of any object can be found by indexing
    * from the rule number and the object number within the rule.
    * The actual representation of the state is determined by the rule. Lists
    * and Maps are common, but it is arbitrary.
    */
   final List<List> states = new List<List>();
@@ skipping 595 matching lines @@
  */
 class DesignatedRuleForObject {
   Function rulePredicate;
   final target;
 
   DesignatedRuleForObject(this.target, this.rulePredicate);
 
   possibleRules(List rules) => rules.filter(rulePredicate);
 }