Updating Polymer to derive from custom elements.

pkg/polymer/test/instance_attrs_test.dart

 // Copyright (c) 2013, 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.
 
 import 'dart:html';
 import 'package:unittest/unittest.dart';
 import 'package:unittest/html_config.dart';
 import 'package:polymer/polymer.dart';
 
 main() {
   useHtmlConfiguration();
 
   test('attributes were deserialized', () {
-    var elem = query('my-element').xtag;
+    var elem = query('my-element');
 
     expect(elem.attributes, {'foo': '123', 'bar': 'hi', 'baz': 'world'},
         reason: 'attributes should be copied to instance');
 
     var text = elem.shadowRoot.text;
     // Collapse adjacent whitespace like a browser would:
     text = text.replaceAll('\n', ' ').replaceAll(new RegExp(r'\s+'), ' ');
 
     expect(text, " foo: 123 bar: hi baz: world ",
         reason: 'text should match expected HTML template');
   });
 }