Adding Element.created constructor

tests/html/custom/attribute_changed_callback_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 attribute_changed_callback_test;
 import 'package:unittest/unittest.dart';
 import 'package:unittest/html_config.dart';
 import 'dart:html';
 
 class A extends HtmlElement {
   static final tag = 'x-a';
   factory A() => new Element.tag(tag);
+  A.created() : super.created();
 
   static var attributeChangedInvocations = 0;
 
   void attributeChanged(name, oldValue, newValue) {
     attributeChangedInvocations++;
   }
 }
 
 class B extends HtmlElement {
   static final tag = 'x-b';
   factory B() => new Element.tag(tag);
+  B.created() : super.created();
 
   static var invocations = [];
 
   void created() {
     invocations.add('created');
   }
 
   void attributeChanged(name, oldValue, newValue) {
     invocations.add('$name: $oldValue => $newValue');
   }
@@ skipping 33 matching lines @@
     b.attributes['data-v'] = 'w';
     B.invocations = [];
     b.attributes['data-v'] = 'x';
     expect(B.invocations, ['data-v: w => x']);
 
     B.invocations = [];
     b.attributes['data-v'] = 'x';
     expect(B.invocations, []);
   });
 }