Migrated test block 21 to Dart 2.0.

tests/corelib_2/regexp/parentheses_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 338 matching lines @@
   var regexp58 = new RegExp(r"a|b(?:[^b])*?c");
   shouldBe(regexp58.firstMatch('badbc'), ['a']);
   var regexp59 = new RegExp(r"(X(?:.(?!X))*?Y)|(Y(?:.(?!Y))*?Z)");
   Expect.listEquals(
       regexp59
           .allMatches('Y aaa X Match1 Y aaa Y Match2 Z')
           .map((m) => m.group(0))
           .toList(),
       ['X Match1 Y', 'Y Match2 Z']);
 }