Update template_binding test that was still using the deprecated mutation_observer polyfill

pkg/template_binding/test/template_binding_test.dart

Index: pkg/template_binding/test/template_binding_test.dart
diff --git a/pkg/template_binding/test/template_binding_test.dart b/pkg/template_binding/test/template_binding_test.dart
index fde463b55f6e83d9d4a4b1a6e674dc9a9b01e20d..cc68bf3200925196e9cf22d5261cb654d7fb6acf 100644
--- a/pkg/template_binding/test/template_binding_test.dart
+++ b/pkg/template_binding/test/template_binding_test.dart
@@ -26,15 +26,9 @@ import 'utils.dart';
 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();