type '(BuildContext, Widget) => StreamBuilder<Locale?>' is not a subtype of type '((BuildContext, Widget?) => Widget)?'

[alexrintt]

Hi! Thanks for your package, it's super complete and useful. I use it in my project, but I found some typing gaps:

Flutter 2.2.0 • channel stable • https://github.com/flutter/flutter.git
Framework • revision b22742018b (2 weeks ago) • 2021-05-14 19:12:57 -0700
Engine • revision a9d88a4d18
Tools • Dart 2.13.0

When I use:

MaterialApp(
  builder: FlutterI18n.rootAppBuider(),
);

It throws an exception:

══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
The following _TypeError was thrown building AppMock(dirty):
type '(BuildContext, Widget) => StreamBuilder<Locale?>' is not a subtype of type '((BuildContext,
Widget?) => Widget)?'

It happens because the package has missing types in many places: lib\flutter_i18n.dart

class FlutterI18n {
  /// ...
 static /** Missing return types **/ rootAppBuilder() {
    return (BuildContext context, Widget child) {
      final instance = _retrieveCurrentInstance(context);
      return StreamBuilder<Locale?>(
          initialData: instance?.locale,
          stream: instance?._localeStream.stream,
          builder: (BuildContext context, AsyncSnapshot<Locale?> snapshot) {
            return Directionality(
              textDirection: _findTextDirection(snapshot.data),
              child: child,
            );
          });
    };
  }
  /// ...
}

I opened a PR to apply the fix for this specific location, _but I would like to discuss adding effective_dart to the whole package and making the entire development flow follow these rules,_ to minimize bugs like this and many others.

_{Anyway, thanks for the hard work and the awesome package!}

[ilteoood]

Thank you for the PR. Any help is REALLY appreciated 😁