Migrated test block 19 to Dart 2.0.

tests/corelib_2/reg_exp4_test.dart

 // Copyright (c) 2011, 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";
 
 main() {
   try {
     RegExp ex = new RegExp(null);
@@ skipping 29 matching lines @@
   }
   try {
     new RegExp(r"^\w+$").stringMatch(null);
     Expect.fail("Expected: ArgumentError got: no exception");
   } catch (ex) {
     if (!(ex is ArgumentError)) {
       Expect.fail("Expected: ArgumentError got: ${ex}");
     }
   }
 }