add mutation observer polyfill package

pkg/custom_element/test/custom_element_test.dart

Index: pkg/custom_element/test/custom_element_test.dart
diff --git a/pkg/custom_element/test/custom_element_test.dart b/pkg/custom_element/test/custom_element_test.dart
index 93fcd1555cc2e63f603c975318a45098bc377977..d5894c2d49bcd95ac7c08f04e50b90a2d0414903 100644
--- a/pkg/custom_element/test/custom_element_test.dart
+++ b/pkg/custom_element/test/custom_element_test.dart
@@ -13,6 +13,16 @@ import 'package:unittest/unittest.dart';
 main() {
   useHtmlConfiguration();
 
+  // Load the MutationObserver polyfill.
+  HttpRequest.getString('/root_dart/pkg/mutation_observer/lib/'
+      'mutation_observer.js').then((code) {
+    document.head.children.add(new ScriptElement()..text = code);
+
+    customElementTests();
+  });
+}
+
+customElementTests() {
   test('register creates the element and calls lifecycle methods', () {
     // Add element to the page.
     var element = new Element.html('<fancy-button>foo bar</fancy-button>');