Migrated test block 19 to Dart 2.0.

tests/corelib_2/regexp/assertion_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 97 matching lines @@
       r"(?!(u|m{0,}g+)u{1,}|2{2,}!1%n|(?!K|(?=y)|(?=ip))+?)(?=(?=(((?:7))*?)*?))p",
       multiLine: true);
   shouldBeNull(regex22.firstMatch('55up'));
 
   var regex23 = new RegExp(r"(?=(a)b|c?)()*d");
   shouldBeNull(regex23.firstMatch('ax'));
 
   var regex24 = new RegExp(r"(?=a|b?)c");
   shouldBeNull(regex24.firstMatch('x'));
 }