Mark streamed_conversion_utf8_decode_test as slow for FF.

tests/lib/lib.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.
 
 [ $compiler == dart2js && $runtime == drt ]
 convert/chunked_conversion_utf87_test: Pass, Fail # v8 bug: Issue 12293
 typed_data/byte_data_test: Pass, Fail # v8 bug: Issue 12293
 
 [ $compiler == dart2js ]
 math/double_pow_test: Fail
@@ skipping 65 matching lines @@
 [ $compiler == dart2dart && $minified ]
 json/json_test: Fail                            # Issue 10961
 
 [ $runtime == ff ]
 # FF setTimeout can fire early: https://bugzilla.mozilla.org/show_bug.cgi?id=291386
 multiple_timer_test: Pass, Fail
 timer_isolate_test: Pass, Fail
 timer_test: Pass, Fail
 convert/chunked_conversion_utf88_test: Pass, Timeout  # Issue 12029
 convert/streamed_conversion_utf8_encode_test: Pass, Timeout  # Issue 12029
+convert/streamed_conversion_utf8_decode_test: Pass, Slow  # Issue 12029
 
 [ $runtime == ie9 ]
 # IE9 doesn't support typed arrays.
 typed_data/*: Fail # Issue 11971
 convert/chunked_conversion_utf88_test: Pass, Slow, Timeout  # Issue 12029
 convert/streamed_conversion_utf8_encode_test: Pass, Timeout  # Issue 12029
 convert/streamed_conversion_json_utf8_decode_test: Pass, Timeout # Issue 12029
 async/deferred/deferred_api_test: Pass, Timeout # Issue 12029
 
 [ $runtime == opera ]
@@ skipping 25 matching lines @@
 [ $compiler == none && $runtime == drt && $system == windows ]
 async/multiple_timer_test: Fail, Pass # See Issue 10982
 async/timer_test: Fail, Pass # See Issue 10982
 
 [ $arch == simmips || $arch == mips ]
 convert/chunked_conversion_utf88_test: Pass, Slow # Issue 12025.
 convert/streamed_conversion_json_utf8_decode_test: Pass, Slow
 
 [ $arch == simarm ]
 convert/chunked_conversion_utf88_test: Pass, Slow # Issue 12644.