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

pkg/serialization/test/polyfill_identity_map_test.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.
 
 /**
  * Provide a trivial test for identity based hashed collections, which we
  * provide here until an implementation is available in the regular libraries.
  */
 // TODO(alanknight): Remove once identity-hashed collections are available.
 // Issue 4161.
 library identity_set_test;
 
 import '../../unittest/lib/unittest.dart';
-import '../lib/src/polyfill_identity_set.dart';
+import '../lib/src/serialization_helpers.dart';
 
 class Foo {
   var x;
   Foo(this.x);
   int get hashCode => x.hashCode;
   bool operator ==(a) => a.x == x;
 }
 
 main() {
   test('basic', () {
     var one = new Foo(3);
     var two = new Foo(3);
     var map = new Map();
     var identityMap = new IdentityMap();
     map[one] = one;
     map[two] = two;
     identityMap[one] = one;
     identityMap[two] = two;
     expect(map.length, 1);
     expect(identityMap.length, 2);
     for (var each in identityMap.values) {
       expect(each, one);
     }
   });
+
+  test('uniquing primitives', () {
+    var map = new IdentityMap();
+    var one = 'one';
+    var two = new String.fromCharCodes(one.charCodes);
+    map[one] = 1;
+    expect(map[two], 1);
+    expect(map[one], 1);
+  });
 }