regenerate fixed custom_elements polyfill

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 939fb36ed9f7845cd3d24d267d65d3ae2ddf6cec..413d7d699a04f0e2acb736234d887a811520642e 100644
--- a/pkg/custom_element/lib/polyfill.dart
+++ b/pkg/custom_element/lib/polyfill.dart
@@ -54,12 +54,6 @@ 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);