Adding Element.created constructor

tools/dom/templates/html/dart2js/html_dart2js.darttemplate

Index: tools/dom/templates/html/dart2js/html_dart2js.darttemplate
diff --git a/tools/dom/templates/html/dart2js/html_dart2js.darttemplate b/tools/dom/templates/html/dart2js/html_dart2js.darttemplate
index bf62f8b589e1b471175f08ec9816f1542b9a5295..015b808037b85a5d78e05d1c1fde7161954e39cf 100644
--- a/tools/dom/templates/html/dart2js/html_dart2js.darttemplate
+++ b/tools/dom/templates/html/dart2js/html_dart2js.darttemplate
@@ -119,6 +119,13 @@ ElementList queryAll(String selector) => document.queryAll(selector);
 // Dart issue 1990.
 class HtmlElement extends Element native "HTMLElement" {
   factory HtmlElement() { throw new UnsupportedError("Not supported"); }
+
+  /**
+   * Constructor instantiated by the DOM when a custom element has been created.
+   *
+   * This can only be called by subclasses from their created constructor.
+   */
+  HtmlElement.created(): super.created();
 }
 
 // Support for Send/ReceivePortSync.