Fix pub run tests on Windows.

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

Index: sdk/lib/_internal/pub/test/run/nonexistent_script_test.dart
diff --git a/sdk/lib/_internal/pub/test/run/nonexistent_script_test.dart b/sdk/lib/_internal/pub/test/run/nonexistent_script_test.dart
index cb233b38b4530ea6eea8260ebc8f1ecf1cfcf9c7..03e9d9d7a014e2c0c02f0ced80a10c2d280769e7 100644
--- a/sdk/lib/_internal/pub/test/run/nonexistent_script_test.dart
+++ b/sdk/lib/_internal/pub/test/run/nonexistent_script_test.dart
@@ -2,6 +2,8 @@
 // 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';
@@ -15,7 +17,7 @@ main() {
     ]).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);
   });
 }