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

pkg/serialization/test/polyfill_identity_set_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';
-
-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);
-    }
-  });
-}