Fixed failures caused by corelib_2/regexp/UC16_test on ff

tests/corelib_2/corelib_2.status

 # Copyright (c) 2017, 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.
 
 # All static_tests have expected compile-time errors.
 [ $strong && $compiler != dart2analyzer && $compiler != dartdevc ]
 core_runtime_types_static_test: MissingCompileTimeError
 splay_tree_test/01: MissingCompileTimeError
 splay_tree_test/02: MissingCompileTimeError
 string_base_vm_static_test: MissingCompileTimeError
@@ skipping 230 matching lines @@
 
 [ $compiler == dart2js && $runtime != none && !$dart2js_with_kernel]
 symbol_reserved_word_test/03: RuntimeError # Issue 19972, new Symbol('void') should be allowed.
 regexp/pcre_test: Pass, Slow # Issue 21593
 
 [ ($compiler == none && $runtime == vm) || $compiler == dart2js ]
 string_base_vm_static_test: MissingCompileTimeError
 
 [ $compiler == dart2js && ($runtime == ff || $runtime == jsshell) ]
 string_case_test/01: Fail, OK # Bug in Firefox.
-
-[ $compiler == dart2js && $runtime == jsshell ]
 regexp/UC16_test: RuntimeError
 
 [ $compiler == none && $runtime == drt ]
 string_from_environment2_test: Skip
 string_from_environment3_test: Skip
 string_from_environment_test: Skip
 
 [ $system == windows && $arch == x64 ]
 stopwatch_test: Skip  # Flaky test due to expected performance behaviour.
 
@@ skipping 281 matching lines @@
 splay_tree_from_iterable_test: RuntimeError
 
 [ $compiler == precompiler && $runtime == dart_precompiled && !$checked ]
 splay_tree_from_iterable_test: RuntimeError
 
 [ $runtime == vm || $runtime == dart_precompiled || $runtime == flutter ]
 regexp/pcre_test: Pass, Slow, Timeout
 
 [ $mode == debug ]
 regexp/pcre_test: Pass, Slow # Issue 22008