Can't load asset on flutter integration test

Question

I am writing a integration test in my flutter app, it works if I run app. But when I run integration test class, it seems failed to load json file from asset. This is my integration test file

    void main() {
      IntegrationTestWidgetsFlutterBinding.ensureInitialized();
      group('login test', () {
        testWidgets('full login test', (tester) async {
          app.mainCommon("dev");
          await tester.pumpAndSettle();
        });
      });
    }

and this is how i load json file

configString = await rootBundle.loadString('config/dev.json');