Improve handling of intentionally broken links in test scripts.

tests/_chrome/sample_test.dart

 // Copyright (c) 2013, 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 sample_test;
 import 'package:unittest/unittest.dart';
 import 'package:unittest/html_config.dart';
 import 'dart:_chrome' as _chrome;
 
 main() {
   useHtmlConfiguration();
   test('access', () {
     var window = _chrome.app.window;
     expect(window is _chrome.WindowModule, true);
   });
 
   test('fails from browser', () {
     // APIs should not work in standard browser apps.
     expect(() {
-      _chrome.app.window.create('foo.html');
+      _chrome.app.window.create('IntentionallyMissingFile.html');
     }, throws);
   });
 }