Get binstubs working on Windows.

sdk/lib/_internal/pub/test/global/binstubs/missing_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 '../../descriptor.dart' as d;
 import '../../test_pub.dart';
 
 main() {
   initConfig();
   integration("errors if an executable's script can't be found", () {
     d.dir("foo", [
       d.pubspec({
         "name": "foo",
         "executables": {
           "missing": "not_here",
           "nope": null
         }
       })
     ]).create();
 
     var pub = startPub(args: ["global", "activate", "-spath", "../foo"]);
 
     pub.stderr.expect('Warning: Executable "missing" runs '
-        '"bin/not_here.dart", which was not found in foo.');
+        '"${p.join('bin', 'not_here.dart')}", which was not found in foo.');
     pub.stderr.expect('Warning: Executable "nope" runs '
         '"${p.join('bin', 'nope.dart')}", which was not found in foo.');
     pub.shouldExit();
   });
 }