Make RegExp constructor non-const.

utils/tests/pub/update/hosted/remove_removed_transitive_dependency_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("removes a transitive dependency that's no longer depended on", () {
     servePackages([
       package("foo", "1.0.0", [dependency("shared-dep")]),
       package("bar", "1.0.0", [
         dependency("shared-dep"),
         dependency("bar-dep")
       ]),
       package("shared-dep", "1.0.0"),
       package("bar-dep", "1.0.0")
     ]);
 
     appDir([dependency("foo"), dependency("bar")]).scheduleCreate();
 
     schedulePub(args: ['update'],
-        output: const RegExp(r"Dependencies updated!$"));
+        output: new RegExp(r"Dependencies updated!$"));
 
     packagesDir({
       "foo": "1.0.0",
       "bar": "1.0.0",
       "shared-dep": "1.0.0",
       "bar-dep": "1.0.0",
     }).scheduleValidate();
 
     appDir([dependency("foo")]).scheduleCreate();
 
     schedulePub(args: ['update'],
-        output: const RegExp(r"Dependencies updated!$"));
+        output: new RegExp(r"Dependencies updated!$"));
 
     packagesDir({
       "foo": "1.0.0",
       "bar": null,
       "shared-dep": "1.0.0",
       "bar-dep": null,
     }).scheduleValidate();
 
     run();
   });
 }