Only allow one shadowRoot.

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 573c3545f622ddfa642d0cdc9c2df3671687bbd4..81b796ae1fdd4140ab83b9e3f0c1fa25224fb98a 100644
--- a/sky/framework/sky-element/sky-element.sky
+++ b/sky/framework/sky-element/sky-element.sky
@@ -31,7 +31,7 @@ var BasePrototype = createPrototype(HTMLElement.prototype, {
     if (!this.shadowRoot) {
       var template = templates.get(this.localName);
       if (template) {
-        var shadow = this.createShadowRoot();
+        var shadow = this.ensureShadowRoot();
         shadow.appendChild(template.createInstance(this));
       }
     }