Stop Gradle daemon before attempting to delete Flutter project (#15819)

dev/devicelab/lib/tasks/plugin_tests.dart

@@ -89,6 +89,16 @@ class FlutterProject {
   }
 
   Future<Null> delete() async {
+    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 parent.delete(recursive: true);
   }
 }