Make RegExp constructor non-const.

utils/tests/pub/update/hosted/fail_gracefully_on_url_resolve_test.dart

 // Copyright (c) 2012, 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.
 
 library pub_tests;
 
 import 'dart:io';
 
 import '../../test_pub.dart';
 import '../../../../../pkg/unittest/lib/unittest.dart';
 
 main() {
   test('fails gracefully if the url does not resolve', () {
     dir(appPath, [
       pubspec({
         "name": "myapp",
         "dependencies": {
           "foo": {
             "hosted": {
               "name": "foo",
               "url": "http://pub.invalid"
             }
           }
          }
       })
     ]).scheduleCreate();
 
     schedulePub(args: ['update'],
-        error: const RegExp('Could not resolve URL "http://pub.invalid".'),
+        error: new RegExp('Could not resolve URL "http://pub.invalid".'),
         exitCode: 1);
 
     run();
   });
 }