Reenable mutation_observer_test

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
 */*/*/*/packages/*/*: Skip
 
 # Skip non-test files ending with "_test".
 scheduled_test/lib/*: Skip
 polymer/example/scoped_style/*: Skip
 
 scheduled_test/test/scheduled_server_test: Pass, Slow, Fail # Issue 9231, 13524
 scheduled_test/test/scheduled_process_test: Pass, Slow # Issue 9231
 
 # Skip test not runnable via test.dart
 third_party/html5lib/test/dom_compat_test: Skip
 third_party/html5lib/test/browser/browser_test: Skip
 
-[ $compiler == none && ( $runtime == dartium || $runtime == drt ) ]
-mutation_observer/test/mutation_observer_test: Pass, Fail # Issue 13606
-
 [ $compiler == dart2js ]
 analyzer_experimental/test/generated/ast_test: RuntimeError #Issue 12341
 
 [ $compiler == dart2js && $checked && $runtime == ie9 ]
 crypto/test/base64_test: Timeout # Issue 12486
 
 [ $runtime == d8 || $runtime == jsshell ]
 unittest/test/unittest_nested_groups_setup_teardown_test: Pass, RuntimeError # http://dartbug.com/10109
 stack_trace/test/vm_test: RuntimeError, OK # VM-specific traces
 sequence_zip/test/stream_test: RuntimeError, OK # Timers are not supported.
@@ skipping 182 matching lines @@
 
 # Issue http://dartbug.com/12930
 [ $runtime == vm ]
 intl/test/message_extraction/message_extraction_test: Pass, Fail # Issue 12930
 
 [ $compiler == none && $runtime == vm ]
 observe/test/observe_test: Pass, Fail # Issue 13543
 
 [ $compiler == none && $runtime == vm && $checked ]
 intl/test/message_extraction/failed_extraction_test: Pass, Fail # Issue 13543