Migrated test block 21 to Dart 2.0.

tests/corelib_2/regexp/overflow_test.dart

 // Copyright (c) 2014, the Dart project authors. All rights reserved.
 // Copyright 2013 the V8 project authors. All rights reserved.
 // Copyright (C) 2005, 2006, 2007, 2008, 2009 Apple Inc. All rights reserved.
 //
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions
 // are met:
 // 1.  Redistributions of source code must retain the above copyright
 //     notice, this list of conditions and the following disclaimer.
 // 2.  Redistributions in binary form must reproduce the above copyright
@@ skipping 25 matching lines @@
   shouldBeNull(regexp2.firstMatch(''));
 
   var s3 =
       r"&{6}u4a64YfQP{C}u88c4u5772Qu8693{4294967167}u85f2u7f3fs((uf202){4})u5bc6u1947";
   var regexp3 = new RegExp(s3);
   shouldBeNull(regexp3.firstMatch(s3));
 
   var regexp4 = new RegExp(r"[^a$]{4294967295}");
   shouldBeNull(regexp4.firstMatch(s3));
 }