update elements and fix some bugs

test/iron_meta_test.dart

Index: test/iron_meta_test.dart
diff --git a/test/iron_meta_test.dart b/test/iron_meta_test.dart
index 267861932ba1d73d1262d2ed11313b94413cdf4c..1f619fe47e173fbca29121b78a32bf66b8ab0844 100644
--- a/test/iron_meta_test.dart
+++ b/test/iron_meta_test.dart
@@ -4,6 +4,7 @@
 @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';
@@ -137,5 +138,15 @@ main() async {
         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);
+      });
+    });
   });
 }