update custom elements and shadow dom

pkg/custom_element/lib/polyfill.dart

Index: pkg/custom_element/lib/polyfill.dart
diff --git a/pkg/custom_element/lib/polyfill.dart b/pkg/custom_element/lib/polyfill.dart
index 413d7d699a04f0e2acb736234d887a811520642e..939fb36ed9f7845cd3d24d267d65d3ae2ddf6cec 100644
--- a/pkg/custom_element/lib/polyfill.dart
+++ b/pkg/custom_element/lib/polyfill.dart
@@ -54,6 +54,12 @@ bool get _isReady {
  */
 Future loadCustomElementPolyfill() {
   if (!document.supportsRegister && !js.context.hasProperty('CustomElements')) {
+    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);