Only allow one shadowRoot.

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();
 
 function createPrototype(proto, definition) {
@@ skipping 13 matching lines @@
   },
 
   created: function() {
     // override
   },
 
   attachedCallback: function() {
     if (!this.shadowRoot) {
       var template = templates.get(this.localName);
       if (template) {
-        var shadow = this.createShadowRoot();
+        var shadow = this.ensureShadowRoot();
         shadow.appendChild(template.createInstance(this));
       }
     }
     this.attached();
   },
 
   attached: function() {
     // override
   },
 
@@ skipping 20 matching lines @@
   if (template && template.localName === 'template')
     templates.set(prototype.name, template);
 
   document.registerElement(prototype.name, {
     prototype: createPrototype(BasePrototype, prototype),
   });
 };
 
 module.exports = SkyElement;
 </script>