update custom elements and shadow dom

tests/html/utils.dart

Index: tests/html/utils.dart
diff --git a/tests/html/utils.dart b/tests/html/utils.dart
index b1d7f689911f7d14958f4bbe79d3417259a7ba9d..9b469b1f20c3958c2e0972729789fbc39d0fd8d7 100644
--- a/tests/html/utils.dart
+++ b/tests/html/utils.dart
@@ -166,7 +166,7 @@ Future loadCustomElementPolyfill() {
     var script = new ScriptElement()
         ..src = '/packages/custom_element/custom-elements.debug.js';
     document.head.append(script);
-    return document.body.on['WebComponentsReady'].first;
+    return document.on['WebComponentsReady'].first;
   }
   return new Future.value();
 }