Migrated test block 19 to Dart 2.0.

tests/corelib_2/reg_exp_pattern_test.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 // Dart test for testing regular expressions in Dart.
 
 import "package:expect/expect.dart";
 
 // Regression test for http://dartbug.com/17998
 
 main() {
   for (var s in [
     r"a",
     r"a|b",
     r"(?:)",
     r"^",
     r"$",
     r"^$",
     r"$^",
     r"",
     r"\\",
     r"/",
     r"[^]",
     "\x00",
   ]) {
     Expect.equals(s, new RegExp(s).pattern);
   }
 }