Only allow one shadowRoot.

sky/tests/dom/appendChild.sky

Index: sky/tests/dom/appendChild.sky
diff --git a/sky/tests/dom/appendChild.sky b/sky/tests/dom/appendChild.sky
index 60da8036d7fffb7209712b2b6e57a2818308c8f7..e9025c9beb255debffcda3db81715f528d25bcfe 100644
--- a/sky/tests/dom/appendChild.sky
+++ b/sky/tests/dom/appendChild.sky
@@ -53,7 +53,7 @@ describe("appendChild", function() {
   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.appendChild(doc);
     });