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..6e5010e9d2b949709d5b2232824eb292ca3afa85 100644
--- a/tests/html/utils.dart
+++ b/tests/html/utils.dart
@@ -163,10 +163,15 @@ void validateNodeTree(Node a, Node b, [String path = '']) {
 
 Future loadCustomElementPolyfill() {
   if (!document.supportsRegister) {
+    if (!MutationObserver.supported) {
+      var script = new ScriptElement()
+          ..src = '/packages/mutation_observer/mutation_observer.js';
+      document.head.append(script);
+    }
     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();
 }