Add shadowRootReady callback for SkyElement.

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

Index: sky/framework/sky-element/sky-element.sky
diff --git a/sky/framework/sky-element/sky-element.sky b/sky/framework/sky-element/sky-element.sky
index aaf130faab83431b0f057c303a10ce166511b705..19eb2535f905aa53982923d5f77c242638ccd5eb 100644
--- a/sky/framework/sky-element/sky-element.sky
+++ b/sky/framework/sky-element/sky-element.sky
@@ -44,6 +44,10 @@ class SkyElement extends HTMLElement {
     // override
   }
 
+  shadowRootReady() {
+    // override
+  }
+
   createdCallback() {
     this.created();
   }
@@ -54,6 +58,7 @@ class SkyElement extends HTMLElement {
       if (template) {
         var shadow = this.ensureShadowRoot();
         shadow.appendChild(template.createInstance(this));
+        this.shadowRootReady();
       }
     }
     this.attached();