Add shadowRootReady callback for SkyElement.

sky/framework/sky-element/sky-element.sky

 <!--
 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 -->
 <import src="TemplateBinding.sky" />
 <script>
 var templates = new Map();
 
 class SkyElement extends HTMLElement {
@@ skipping 26 matching lines @@
   }
 
   dettached() {
     // override
   }
 
   attributeChanged(attrName, oldValue, newValue) {
     // override
   }
 
+  shadowRootReady() {
+    // override
+  }
+
   createdCallback() {
     this.created();
   }
 
   attachedCallback() {
     if (!this.shadowRoot) {
       var template = templates.get(this.localName);
       if (template) {
         var shadow = this.ensureShadowRoot();
         shadow.appendChild(template.createInstance(this));
+        this.shadowRootReady();
       }
     }
     this.attached();
   }
 
   dettachedCallback() {
     this.dettached();
   }
 
   attributeChangedCallback(attrName, oldValue, newValue) {
     // reserved for canonical behavior
     this.attributeChanged(attrName, oldValue, newValue);
   }
 };
 
 module.exports = SkyElement;
 </script>