BLoC state management modifications

[vcadillog]

Hi again, great work; I had made some changes for it to match with BLoC pattern; if you think that this can be helpful consider on merging or making a secondary tree, since I made some refactors and adding a controller.
Here you can see how I implemented it with firebase_auth.
https://github.com/vcadillog/flutter_firebase_login_bloc_pattern/blob/master/lib/login/view/login_form.dart
Some things to note when isBlocPattern = true (the flag for bloc). It skips all the error handling logic, since I do it in the BLoC repository layer and for that reason I create a Flushbar myself.

Additional changes included, for example in login_form.dart file:
176 await _submitController.forward();
177 setState(() => _isSubmitting = true);
This code potentially leads to a dispose controller error, because if the user taps fast enought the forgot password screen before the setState takes action it disposes the submitController; so I swapped the order to ensure _isSubmitting is true and disable undesired gestures.

[juliansteenbakker]

Thank you for your PR, however i won't be merging this, because it will add lots of boiler plate code for a specific plugin (BLOC) that not all people use.

[artem-bakuta]

Would be great to have something inside

[artem-bakuta]

I have tried to use Completer<String?> to deliver future into onLogin, but it always throw me:

Unhandled Exception: Navigator operation requested with a context that does not include a Navigator.
The context used to push or pop routes from the Navigator must be that of a widget that is a descendant of a Navigator widget.

So solution out of the box should be great solution