Only allow one shadowRoot.

sky/benchmarks/dom/creation.sky

 <sky>
 <import src="../resources/runner.sky" as="PerfRunner" />
 <script>
 var sky = document.querySelector("sky");
 
 var widgets = 0;
 var basicElements = 0;
 var texts = 0;
 
 var WidgetPrototype = Object.create(HTMLElement.prototype);
 WidgetPrototype.createdCallback =  function() {
   widgets++;
   this.wasCreated = true;
   this.wasAttached = false;
   this.wasDetached = false;
   this.attrsChanged = [];
-  this.createShadowRoot();
+  this.ensureShadowRoot();
 };
 
 WidgetPrototype.attachedCallback = function() {
   this.wasAttached = true;
 };
 
 WidgetPrototype.detachedCallback = function() {
   this.wasDetached = true;
 };
 
@@ skipping 57 matching lines @@
   // console.log("widgets: " + widgets);
   // console.log("basic elements: " + basicElements);
   // console.log("texts: " + texts);
   // CONSOLE: LOG: widgets: 340
   // CONSOLE: LOG: basic elements: 1021
   // CONSOLE: LOG: texts: 1020
   done();
 });
 </script>
 </sky>