remove xtag from Polymer

pkg/polymer/example/component/news/test/news_index_test.dart

Index: pkg/polymer/example/component/news/test/news_index_test.dart
diff --git a/pkg/polymer/example/component/news/test/news_index_test.dart b/pkg/polymer/example/component/news/test/news_index_test.dart
index 77713e03f1e13374b7f0137673580e76fd880de6..034894bb58b0814a31c12c9f51c4c81ebfe6f16a 100644
--- a/pkg/polymer/example/component/news/test/news_index_test.dart
+++ b/pkg/polymer/example/component/news/test/news_index_test.dart
@@ -22,8 +22,7 @@ main() {
     final items = listComp.queryAll('li');
     expect(items.length, 6);
     expect(extractLinks(items), ['1', '2', '3', '4', '4', '5']);
-    expect(listComp.xtag is Polymer, true,
-        reason: 'x-news should be created');
+    expect(listComp is Polymer, true, reason: 'x-news should be created');
 
     final contents = listComp.shadowRoot.queryAll('content');
     expect(contents.length, 2, reason: 'news has 2 content tags');