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

pkg/serialization/test/polyfill_identity_map_test.dart

Index: pkg/serialization/test/polyfill_identity_map_test.dart
diff --git a/pkg/serialization/test/polyfill_identity_set_test.dart b/pkg/serialization/test/polyfill_identity_map_test.dart
similarity index 79%
rename from pkg/serialization/test/polyfill_identity_set_test.dart
rename to pkg/serialization/test/polyfill_identity_map_test.dart
index b44d2cd3a0e7cce1d5a2c841675d237766b2f744..edb7b20b17f4e9e31fbf9ed9ce8a21cfe848c1f2 100644
--- a/pkg/serialization/test/polyfill_identity_set_test.dart
+++ b/pkg/serialization/test/polyfill_identity_map_test.dart
@@ -11,7 +11,7 @@
 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;
@@ -36,4 +36,13 @@ main() {
       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);
+  });
 }