Update template_binding test that was still using the deprecated mutation_observer polyfill

pkg/template_binding/test/template_binding_test.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library template_binding.test.template_binding_test;
 
 import 'dart:async';
 import 'dart:html';
 import 'dart:js' show JsObject;
 import 'dart:math' as math;
 import 'package:observe/observe.dart';
 import 'package:template_binding/template_binding.dart';
 import 'package:unittest/html_config.dart';
 import 'package:unittest/unittest.dart';
 
 // TODO(jmesserly): merge this file?
 import 'binding_syntax.dart' show syntaxTests;
 import 'utils.dart';
 
 // Note: this file ported from TemplateBinding's tests/tests.js
 
 // TODO(jmesserly): submit a small cleanup patch to original. I fixed some
 // cases where "div" and "t" were unintentionally using the JS global scope;
 // look for "assertNodesAre".
 
 main() => dirtyCheckZone().run(() {
   useHtmlConfiguration();
 
-  // Load MutationObserver polyfill in case IE needs it.
-  var script = new ScriptElement()
-      ..src = '/root_dart/pkg/mutation_observer/lib/mutation_observer.min.js';
-  var polyfillLoaded = script.onLoad.first;
-  document.head.append(script);
-
-  setUp(() => polyfillLoaded.then((_) {
+  setUp(() {
     document.body.append(testDiv = new DivElement());
-  }));
+  });
 
   tearDown(() {
     testDiv.remove();
     clearAllTemplates(testDiv);
     testDiv = null;
   });
 
   test('MutationObserver is supported', () {
     expect(MutationObserver.supported, true, reason: 'polyfill was loaded.');
   });
@@ skipping 2682 matching lines @@
 class TestAccessorModel extends Observable {
   @observable var value = 1;
   var count = 0;
 
   @reflectable
   get prop {
     count++;
     return value;
   }
 }