Adding Element.created constructor

tests/html/custom/attribute_changed_callback_test.dart

Index: tests/html/custom/attribute_changed_callback_test.dart
diff --git a/tests/html/custom/attribute_changed_callback_test.dart b/tests/html/custom/attribute_changed_callback_test.dart
index 955399f3abe7dad56152e43df0a8310e4abe4533..7a10e1d5d6c5340bdf33997c0dd5c5f83d7f3c87 100644
--- a/tests/html/custom/attribute_changed_callback_test.dart
+++ b/tests/html/custom/attribute_changed_callback_test.dart
@@ -10,6 +10,7 @@ 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;
 
@@ -21,6 +22,7 @@ class A extends HtmlElement {
 class B extends HtmlElement {
   static final tag = 'x-b';
   factory B() => new Element.tag(tag);
+  B.created() : super.created();
 
   static var invocations = [];