Fix ShadowRoot constructor for dart2js

lib/dom/templates/html/frog/factoryprovider_ShadowRoot.darttemplate

Index: lib/dom/templates/html/frog/factoryprovider_ShadowRoot.darttemplate
diff --git a/lib/dom/templates/dom/dummy/factoryprovider.darttemplate b/lib/dom/templates/html/frog/factoryprovider_ShadowRoot.darttemplate
similarity index 71%
copy from lib/dom/templates/dom/dummy/factoryprovider.darttemplate
copy to lib/dom/templates/html/frog/factoryprovider_ShadowRoot.darttemplate
index 68fc08702ba52cba199fe40319ffa1fdf1549381..f4c6f79a2a2958bb67dd5814f2f28330a84119e4 100644
--- a/lib/dom/templates/dom/dummy/factoryprovider.darttemplate
+++ b/lib/dom/templates/html/frog/factoryprovider_ShadowRoot.darttemplate
@@ -3,5 +3,7 @@
 // BSD-style license that can be found in the LICENSE file.
 
 class $FACTORYPROVIDER {
-  factory $CONSTRUCTOR($PARAMETERS) => _dummy();
+  factory ShadowRoot(Element host) native '''
+      return new WebKitShadowRoot(host);
+    ''';
 }