web_components: fix extending another custom element

pkg/web_components/test/interop_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.
 
 library template_wrappers_test;
 
 import 'dart:html';
 import 'dart:async';
 import 'dart:js' show context, JsObject;
 import 'package:unittest/html_config.dart';
@@ skipping 102 matching lines @@
     context.callMethod('addC');
     var list = document.querySelectorAll('x-c');
     expect(list.length, 2);
     expect(list[0], c);
     c = list[1];
     expect(c is HtmlElement, isTrue, reason: 'x-c is HtmlElement');
     expect(c is XCWrapper, isTrue, reason: 'x-c is upgraded to XCWrapper');
     expect(c.x, 7);
     expect(c.wrapperCount, 6);
   });
+
+  test('element can extend another element', () {
+    registerDartType('x-e', XEWrapper);
+    context.callMethod('addE');
+
+    var e = document.querySelector('x-e');
+    expect(e is XEWrapper, isTrue);
+    expect(e.x, 8);
+    expect(e.y, 9);
+  });
 }
 int _count = 0;
 
 abstract class Wrapper {
   int wrapperCount = _count++;
   int get x => _readX(this);
 }
 
 _readX(e) => new JsObject.fromBrowserObject(e)['x'];
 
 class XAWrapper extends HtmlElement with Wrapper {
   XAWrapper.created() : super.created();
 }
 
 class XBWrapper extends DivElement with Wrapper {
   XBWrapper.created() : super.created();
 }
 
 class XCWrapper extends HtmlElement with Wrapper {
   XCWrapper.created() : super.created();
 }
 
 class XDWrapper extends HtmlElement with Wrapper {
   XDWrapper.created() : super.created();
 }
+
+class XEWrapper extends HtmlElement with Wrapper {
+  XEWrapper.created() : super.created();
+
+  int get y => new JsObject.fromBrowserObject(this)['y'];
+}