Only allow one shadowRoot.

sky/tests/dom/replaceChild.sky

Index: sky/tests/dom/replaceChild.sky
diff --git a/sky/tests/dom/replaceChild.sky b/sky/tests/dom/replaceChild.sky
index 17cc7ad7bb21360e9bde8f800122841bb78e1410..bfa1d24262027cb68a14e076abab3b72db8f3f3e 100644
--- a/sky/tests/dom/replaceChild.sky
+++ b/sky/tests/dom/replaceChild.sky
@@ -67,7 +67,7 @@ describe("replaceChild", 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.replaceChild(doc);
     });