Migrated test block 19 to Dart 2.0.

tests/corelib_2/regexp/backreferences_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 27 matching lines @@
   shouldBeFalse(new RegExp(r"\2(...)$").hasMatch('abcabc'));
   shouldBeFalse(new RegExp(r"\2(...)$").hasMatch('abc'));
   shouldBeTrue(new RegExp(r"\1?(...)$").hasMatch('abc'));
   shouldBeTrue(new RegExp(r"\1?(...)$").hasMatch('abc'));
 
   var re = new RegExp(r"[^b]*((..)|(\2))+Sz", caseSensitive: false);
 
   shouldBeFalse(re.hasMatch('axabcd'));
   shouldBeTrue(re.hasMatch('axabcsz'));
 }