fix custom elements that extend tags like "section"

tests/html/custom/constructor_calls_created_synchronously_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 constructor_calls_created_synchronously_test;
 import 'package:unittest/unittest.dart';
 import 'package:unittest/html_config.dart';
 import 'dart:html';
 import '../utils.dart';
 import 'dart:mirrors';
@@ skipping 31 matching lines @@
 
   test('clone node', () {
     A.ncallbacks = 0;
 
     var a = new A();
     expect(A.ncallbacks, 1);
     var b = a.clone(false);
     expect(A.ncallbacks, 2);
     expect(b is A, isTrue);
   });
+
+  test("can extend elements that don't have special prototypes", () {
+    document.register('fancy-section', FancySection, extendsTag: 'section');
+    var fancy = document.createElement('section', 'fancy-section');
+    expect(fancy is FancySection, true, reason: 'fancy-section was registered');
+    expect(fancy.wasCreated, true, reason: 'FancySection ctor was called');
+  });
 }
+
+class FancySection extends HtmlElement {
+  bool wasCreated = false;
+  FancySection.created() : super.created() {
+    wasCreated = true;
+  }
+}