Get binstubs working on Windows.

sdk/lib/_internal/pub_generated/test/global/binstubs/missing_script_test.dart

 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();
   });
 }