Adding annotation for partial browser support and example implementation for MutationObservers.

sdk/lib/html/src/dart2js_MutationObserverSupported.dart

Index: sdk/lib/html/src/dart2js_MutationObserverSupported.dart
diff --git a/sdk/lib/html/src/dart2js_MutationObserverSupported.dart b/sdk/lib/html/src/dart2js_MutationObserverSupported.dart
deleted file mode 100644
index 87fe26b3bff39f99e9efb55d8dfd1239cbd1bbeb..0000000000000000000000000000000000000000
--- a/sdk/lib/html/src/dart2js_MutationObserverSupported.dart
+++ /dev/null
@@ -1,12 +0,0 @@
-// Copyright (c) 2012, 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.
-
-part of html;
-
-/**
- * Checks to see if the mutation observer API is supported on the current
- * platform.
- */
-bool _isMutationObserverSupported() =>
-  JS('bool', '!!(window.MutationObserver || window.WebKitMutationObserver)');