Download to a temp dir and rename on success.

utils/tests/pub/update/pub_update_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/unittest.dart';
@@ skipping 70 matching lines @@
           libPubspec('foo', '0.0.0-not.used')
         ])
       ])
     ]).scheduleCreate();
 
     dir(appPath, [
       pubspec({"name": "myapp", "dependencies": {"foo": {"sdk": "foo"}}})
     ]).scheduleCreate();
 
     schedulePub(args: ['update'],
-        output: '''
-        Dependencies updated!
-        ''');
+        output: const RegExp(r"Dependencies updated!$"));
 
     packagesDir({"foo": null}).scheduleValidate();
 
     run();
   });
 }