Make RegExp's constructor non-const.

utils/tests/pub/install/git/check_out_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('checks out a package from Git', () {
     ensureGit();
 
     git('foo.git', [
       libDir('foo'),
       libPubspec('foo', '1.0.0')
     ]).scheduleCreate();
 
     appDir([{"git": "../foo.git"}]).scheduleCreate();
 
     schedulePub(args: ['install'],
-        output: const RegExp(r"Dependencies installed!$"));
+        output: new RegExp(r"Dependencies installed!$"));
 
     dir(cachePath, [
       dir('git', [
         dir('cache', [gitPackageRepoCacheDir('foo')]),
         gitPackageRevisionCacheDir('foo')
       ])
     ]).scheduleValidate();
 
     dir(packagesPath, [
       dir('foo', [
         file('foo.dart', 'main() => "foo";')
       ])
     ]).scheduleValidate();
 
     run();
   });
 }