Migrated test block 19 to Dart 2.0.

tests/corelib_2/regexp/UC16_test.dart

 // Copyright (c) 2014, the Dart project authors. All rights reserved.
 // Copyright 2008 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
@@ skipping 51 matching lines @@
       [longUC16String, longUC16String.substring(1, 4)],
       "backref-UC16-twice");
 
   assertFalse(new RegExp(r"\xc1", caseSensitive: false).hasMatch('fooA'),
       "quickcheck-uc16-pattern-ascii-subject");
   assertFalse(new RegExp(r"[\xe9]").hasMatch('i'),
       "charclass-uc16-pattern-ascii-subject");
   assertFalse(new RegExp(r"\u5e74|\u6708").hasMatch('t'),
       "alternation-uc16-pattern-ascii-subject");
 }