add mutation observer polyfill package

pkg/pkg.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.
 
 # Don't run any test-like files that show up in packages directories. It
 # shouldn't be necessary to run "pub install" in these packages, but if you do
 # it shouldn't break the tests.
 */packages/*/*: Skip
 */*/packages/*/*: Skip
 */*/*/packages/*/*: Skip
@@ skipping 23 matching lines @@
                                 # overflow in JS (numbers slightly larger than
                                 # 32 bits where we do bitwise operations).
 
 [ $runtime == opera && $compiler == dart2js ]
 intl/test/find_default_locale_browser_test: Fail
 intl/test/date_time_format_http_request_test: Skip # Timeout.
 
 [ $runtime == ie9 ]
 intl/test/date_time_format_http_request_test: Fail # Issue 8983
 mime/test/mime_multipart_transformer_test: Skip # No typed_data on IE9.
-custom_element: Fail # No Mutation Observers on IE9.
 
 [ $runtime == safari ]
 fixnum/test/int_64_test: Pass, Fail # Bug in JSC.
 crypto/test/hmac_sha1_test: Fail # Issue 11407.
 crypto/test/sha1_test: Fail # Issue 11407.
 
 # Skip browser-specific tests on VM
 [ $runtime == vm ]
 path/test/browser_test: Fail, OK # Uses dart:html
 intl/test/find_default_locale_browser_test: Skip
@@ skipping 116 matching lines @@
 serialization/test/no_library_test: Skip # Expected Failure
 
 # Skip tests on the VM if the package depends on dart:html
 [ $runtime == vm ]
 custom_element: Skip
 mdv: Skip
 
 [ $runtime == safari || $runtime == chrome || $runtime == ie9 || $runtime == ff || $runtime == dartium || $runtime == drt ]
 docgen/test/single_library_test: Skip # Uses dart:io