Fix pub run tests on Windows.

sdk/lib/_internal/pub/test/run/nonexistent_script_test.dart

 // Copyright (c) 2014, 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.
 
+import 'package:path/path.dart' as p;
+
 import '../../lib/src/exit_codes.dart' as exit_codes;
 import '../descriptor.dart' as d;
 import '../test_pub.dart';
 import 'utils.dart';
 
 main() {
   initConfig();
   integration('Errors if the script does not exist.', () {
     d.dir(appPath, [
       d.appPubspec()
     ]).create();
 
     var pub = pubRun(args: ["script"]);
-    pub.stderr.expect("Could not find bin/script.dart.");
+    pub.stderr.expect("Could not find ${p.join("bin", "script.dart")}.");
     pub.shouldExit(exit_codes.NO_INPUT);
   });
 }