Only allow one shadowRoot.

sky/tests/dom/replaceChild.sky

 <sky>
 <import src="../resources/chai.sky" />
 <import src="../resources/mocha.sky" />
 <import src="../resources/dom-utils.sky" as="DomUtils" />
 <script>
 describe("replaceChild", function() {
   var childElementCount = DomUtils.childElementCount;
   var childNodeCount = DomUtils.childNodeCount;
 
   it("should throw with invalid arguments", function() {
@@ skipping 49 matching lines @@
     assert.equal(child4.parentNode, parent);
     assert.isNull(oldChild.parentNode);
     assert.equal(childNodeCount(parent), 5);
     assert.equal(childElementCount(parent), 3);
     assert.equal(parent.lastChild, lastChild);
   });
 
   it("should throw when inserting a tree scope", function() {
     var parent = document.createElement("div");
     var doc = new Document();
-    var shadowRoot = document.createElement("span").createShadowRoot();
+    var shadowRoot = document.createElement("span").ensureShadowRoot();
     assert.throws(function() {
       parent.replaceChild(doc);
     });
     assert.throws(function() {
       parent.replaceChild(shadowRoot);
     });
     assert.throws(function() {
       doc.replaceChild(fragment);
     });
   });
 
   it("should throw when appending to a text", function() {
     var parent = new Text();
     assert.throws(function() {
       parent.replaceChild(document.createElement("div"), null);
     });
   });
 });
 </script>
 </sky>