Fix path to .bat file (#15824)

diff --git a/dev/devicelab/lib/tasks/plugin_tests.dart b/dev/devicelab/lib/tasks/plugin_tests.dart
index 418159c..0332895 100644
--- a/dev/devicelab/lib/tasks/plugin_tests.dart
+++ b/dev/devicelab/lib/tasks/plugin_tests.dart
@@ -92,12 +92,9 @@
     if (Platform.isWindows) {
       // A running Gradle daemon might prevent us from deleting the project
       // folder on Windows.
-      await inDirectory(
-        new Directory(path.join(rootPath, 'android')),
-        () async {
-          exec('gradlew.bat', <String>['--stop'], canFail: true);
-        },
-      );
+      await inDirectory(new Directory(rootPath), () async {
+        exec(path.join('android', 'gradlew.bat'), <String>['--stop'], canFail: true);
+      });
     }
     await parent.delete(recursive: true);
   }