update elements and fix some bugs

test/iron_meta_test.dart

 // Copyright (c) 2015, 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.
 @TestOn('browser')
 library polymer_elements.test.iron_meta_test;
 
+import 'dart:js';
 import 'package:polymer_elements/iron_meta.dart';
 import 'package:test/test.dart';
 import 'package:web_components/web_components.dart';
 import 'common.dart';
 
 main() async {
   await initWebComponents();
 
   group('<iron-meta>', () {
     group('basic behavior', () {
@@ skipping 113 matching lines @@
 
       test('let the last value win registration against the key', () {
         var registeredValue = metaPair[0].byKey(metaPair[0].key);
         var firstValue = metaPair[0].value;
         var secondValue = metaPair[1].value;
 
         expect(registeredValue, isNot(firstValue));
         expect(registeredValue, secondValue);
       });
     });
+    
+    group('singleton', () {
+
+      test('only one ironmeta created', () {
+        var ironMeta = context['Polymer']['IronMeta'];
+        var first = ironMeta.callMethod('getIronMeta');
+        var second = ironMeta.callMethod('getIronMeta');
+        expect(first, second);
+      });
+    });
   });
 }