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

tests/html/html.status

 # 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.
 
 window_open_test: Skip # http://dartbug.com/5151
 
 event_test: Skip  # Issue 1996
 webgl_1_test: Skip  # Issue 1495
 xmldocument_test: Skip  # XML in dart:html is incomplete
 xmlelement_test: Skip  # XML in dart:html is incomplete
@@ skipping 70 matching lines @@
 input_element_test/date: Fail            # IE10 does not support this input type.
 input_element_test/datetime: Fail        # IE10 does not support this input type.
 input_element_test/datetime-local: Fail  # IE10 does not support this input type.
 input_element_test/month: Fail           # IE10 does not support this input type.
 input_element_test/time: Fail            # IE10 does not support this input type.
 input_element_test/week: Fail            # IE10 does not support this input type.
 isolates_test: Skip
 localstorage_test: Fail
 measurement_test: Fail, Pass
 messageevent_test: Fail
-mutationobserver_test: Fail
+mutationobserver_test/supported: Fail
 serialized_script_value_test: Fail
 storage_test: Fail, Pass
 svgelement_test/additionalConstructors: Fail
 svgelement2_test: Fail
 svg_3_test: Fail
 typed_arrays_1_test: Fail
 typed_arrays_arraybuffer_test: Fail
 typed_arrays_dataview_test: Fail
 url_test: Fail
 websql_test: Fail
@@ skipping 39 matching lines @@
 indexeddb_4_test: Fail
 input_element_test/date: Fail            # IE9 does not support this input type.
 input_element_test/datetime: Fail        # IE9 does not support this input type.
 input_element_test/datetime-local: Fail  # IE9 does not support this input type.
 input_element_test/month: Fail           # IE9 does not support this input type.
 input_element_test/range: Fail           # IE9 does not support this input type.
 input_element_test/time: Fail            # IE9 does not support this input type.
 input_element_test/week: Fail            # IE9 does not support this input type.
 
 messageevent_test: Fail
-mutationobserver_test: Fail
+mutationobserver_test/supported: Fail
 postmessage_structured_test: Skip   # BUG(5685): times out.
 serialized_script_value_test: Fail
 shadow_dom_layout_test: Fail
 shadow_dom_test: Fail
 storage_test: Fail
 svg_3_test: Fail
 svgelement_test/additionalConstructors: Fail
 svgelement2_test: Fail
 url_test: Fail              # IE9 does not support createObjectURL (it is supported in IE10)
 websocket_test: Fail
@@ skipping 57 matching lines @@
 element_test/constructors: Fail
 element_test/elements: Fail
 element_test/eventListening: Crash
 element_test/queryAll: Fail
 element_webkit_test: Fail
 exceptions_test: Fail
 indexeddb_1_test: Fail
 indexeddb_2_test: Fail
 indexeddb_3_test: Fail
 indexeddb_4_test: Fail
-mutationobserver_test: Fail
+mutationobserver_test/supported: Fail
 performance_api_test: Fail
 serialized_script_value_test: Fail
 svg_3_test: Fail
 svgelement_test/additionalConstructors: Fail
 svgelement2_test: Fail
 typed_arrays_1_test: Fail
 typed_arrays_arraybuffer_test: Fail
 url_test: Fail
 websocket_test: Fail
 fileapi_test: Skip # Timeout.
@@ skipping 69 matching lines @@
 typed_arrays_1_test: Fail # dart2js doesn't support is checks with generic types.
 
 [ $compiler == dart2js && $runtime == ff && $system == linux]
 indexeddb_3_test: Fail  # Caught Error: DATA_ERR: DOM IDBDatabase Exception 5 Error: The data provided does not meet the requirements of the function.
 
 [ $compiler == dart2js && $runtime == ff ]
 xhr_test: Skip # Timeout
 inner_frame_test: Skip # Timeout
 svg_3_test: Skip # Timeout
 documentfragment_test: Skip # Timeout