Flutter: how to test navigation

I have an alert dialog like this:

return AlertDialog(
      actions: <Widget>[
      FlatButton(
      onPressed: (){
       Navigator.pop(context);
      });

I would like to test that, when the user presses on the FlatButton, the alert will pop. This is my test:

await tester.pumpWidget(generateView());
await tester.pump();
await tester.tap(find.byType(FlatButton));
await tester.pump();
expect(find.byType(AlertDialog), findsNothing);

This test will enter inside onPressed(), but it will fail saying that there is still one AlertDialog.