Migrated test block 23 to Dart 2.0.

tests/corelib_2/regexp/quantified-assertions_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 26 matching lines @@
   shouldBeTrue(regexp.hasMatch('a'));
 
   regexp = new RegExp(r"(?!a){0}", multiLine: true);
   shouldBeTrue(regexp.hasMatch('b'));
 
   regexp = new RegExp(r"(?!a){1}", multiLine: true);
   shouldBeTrue(regexp.hasMatch('b'));
 
   shouldBeTrue(new RegExp(r"^(?=a)?b$").hasMatch("b"));
 }