Fix status to mark StringConcatTest as failing in unchecked mode

tests/language/language.status

 # Copyright (c) 2011, 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.
 
 # This directory contains tests that are intended to show the
 # current state of the language.
 
 # In order to maintain maximum test coverage for all builds,
 # please use the following procedure to mark a test
 # failed on architectures other than the one you are working on.
@@ skipping 306 matching lines @@
 ImplicitThisTest/02: Fail # Issue 376
 ImplicitThisTest/03: Fail # Issue 376
 ImplicitThisTest/04: Fail # Issue 376
 ImplicitThisTest/none: Fail # Issue 376
 
 [ ($component == frogsh ||  $component == frog || $component == leg || $component == frogium) && $unchecked ]
 PrivateMemberTest/01: Fail # test seems bad - see issue #353
 PrivateMemberTest/02: Fail # test seems bad - see issue #353
 PrivateMemberTest/03: Fail # test seems bad - see issue #353
 PrivateMemberTest/04: Fail # test seems bad - see issue #353
+StringConcatTest: Fail
 
 [ ($component == frogsh ||  $component == frog || $component == leg || $component == frogium) && $checked ]
 PrivateMemberTest/none: Fail # test seems bad - see issue #353
 
 [ $component == leg ]
 FinalVarNegativeTest: Fail
 StaticTopLevelTest/00: Fail
 StaticTopLevelTest/03: Fail
 StringInterpolate2Test: Fail
 
@@ skipping 10 matching lines @@
 StaticField2RunNegativeTest: Fail
 
 [ $arch == x64 ]
 *: Skip
 
 [ $arch == simarm ]
 *: Skip
 
 [ $arch == arm ]
 *: Skip