Adding Element.created constructor

tools/dom/templates/html/impl/impl_Element.darttemplate

Index: tools/dom/templates/html/impl/impl_Element.darttemplate
diff --git a/tools/dom/templates/html/impl/impl_Element.darttemplate b/tools/dom/templates/html/impl/impl_Element.darttemplate
index 5732c1ebd5469464a8cdc917a75156394a97d705..0e76f3819dc4742d21490654af70601567797850 100644
--- a/tools/dom/templates/html/impl/impl_Element.darttemplate
+++ b/tools/dom/templates/html/impl/impl_Element.darttemplate
@@ -334,6 +334,30 @@ $(ANNOTATIONS)abstract class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
   }
 
   /**
+   * Custom element creation constructor.
+   *
+   * This constructor is used by the DOM when a custom element has been
+   * created. It can only be invoked by subclasses of Element from
+   * that classes created constructor.
+   *
+   *     class CustomElement extends Element {
+   *       factory CustomElement() => new Element.tag('x-custom');
+   *
+   *       CustomElement.created(): super.created() {
+   *          // Perform any element initialization.
+   *       }
+   *     }
+   *     document.register('x-custom', CustomElement);
+   */
+  Element.created(): super._created() {
+    // Validate that this is a custom element & perform any additional
+    // initialization.
+    _initializeCustomElement(this);
+
+    createdCallback();
+  }
+
+  /**
    * Creates the HTML element specified by the tag name.
    *
    * This is similar to [Document.createElement].
@@ -678,7 +702,7 @@ $(ANNOTATIONS)abstract class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
    * Called by the DOM when this element has been instantiated.
    */
   @Experimental()
-  void created() {}
+  void createdCallback() {}
 
   /**
    * Called by the DOM when this element has been inserted into the live