Mark convert/streamed_conversion_utf8_decode_test as slow on firefox

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.
 
 [ $checked && $compiler != dartanalyzer && $compiler != dart2analyzer && $runtime != dartium ]
 mirrors/regress_16321_test/01: CompileTimeError # Issue 16351
 
 [ $checked && $compiler == none && ($runtime == dartium || $runtime == drt) ]
 mirrors/regress_16321_test/01: RunTimeError # Issue 16351
 
@@ skipping 189 matching lines @@
 convert/json_lib_test: Fail # Issue 10961
 
 [ $compiler == dart2js && $minified ]
 mirrors/typedef_test/01: Fail # http://dartbug.com/6490
 mirrors/mirrors_used_get_name_test: RuntimeError
 mirrors/mirrors_used_get_name2_test: RuntimeError
 
 [ $runtime == ff ]
 # FF setTimeout can fire early: https://bugzilla.mozilla.org/show_bug.cgi?id=291386
 convert/streamed_conversion_utf8_decode_test: Pass, Slow  # Issue 12029
+convert/streamed_conversion_utf8_encode_test: Pass, Slow # Issue 12029
 mirrors/mirrors_reader_test: Timeout, Slow, RuntimeError # Issue 16589
 
 [ $runtime == ie9 ]
 # IE9 doesn't support typed arrays.
 typed_data/*: Fail # Issue 11971
 convert/chunked_conversion_utf88_test: Skip  # Issue 12029
 convert/streamed_conversion_json_utf8_decode_test: Skip # Issue 12029
 async/deferred/*: Skip # http://dartbug.com/12635
 mirrors/deferred*: Pass, Fail # http://dartbug.com/12635
 convert/streamed_conversion_utf8_decode_test: Skip # http://dartbug.com/12768
@@ skipping 87 matching lines @@
 [ $compiler == dart2js && $mode == debug ]
 mirrors/native_class_test: Pass, Slow
 
 [ $arch == simmips || $arch == mips ]
 convert/chunked_conversion_utf88_test: Skip  # Pass, Slow Issue 12025.
 convert/streamed_conversion_json_utf8_decode_test: Skip  # Pass, Slow
 
 [ $arch == simarm ]
 convert/chunked_conversion_utf88_test: Skip  # Pass, Slow Issue 12644.
 convert/utf85_test: Skip  # Pass, Slow Issue 12644.