Port sky-scrollable to Dart

sky/framework/sky-element.sky

 <!--
 // Copyright 2015 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.
 -->
 <script>
 import "dart:mirrors";
 import "dart:sky";
 
 class _Registration {
@@ skipping 29 matching lines @@
     // Invoke attributeChanged callback when element is first created too.
     for (Attr attribute in getAttributes())
       attributeChangedCallback(attribute.name, null, attribute.value);
   }
 
   attachedCallback() {
     if (shadowRoot == null) {
       var registration = _registery[tagName];
       if (registration.template != null) {
         ShadowRoot shadow = ensureShadowRoot();
-        var tree = registration.template.content.cloneNode(deep:true);
-        shadow.appendChild(tree);
+        Node content = registration.template.content;
+        shadow.appendChild(document.importNode(content, deep: true));
         shadowRootReady();
       }
     }
     attached();
   }
 
   detachedCallback() {
     detached();
   }
 
@@ skipping 12 matching lines @@
   if (!mirror.isSubclassOf(reflectClass(SkyElement)))
     throw new UnsupportedError('@Tagname can only be used on descendants of SkyElement');
 
   String tagName = _getTagName(type);
   Element template = definition.querySelector('template');
 
   document.registerElement(tagName, type);
   _registery[tagName] = new _Registration(template);
 }
 </script>