main.dart

void main() async {
  // ...
  runApp(MultiProvider(
    providers: [
      Provider<Api>(create: (_) => Api()),
      FutureProvider<UserService>(
        create: (context) async {
          var service = UserService(Provider.of(context, listen: false));
          await service.init();
          return service;
        }
      ),
      AwaitProvider<UserService>(),
      ChangeNotifierProvider<ChatService>(
        create: (context) => ChatService(Provider.of<UserService>(context, listen: false)), // WORKS!
      )
    ],
  // ...
}


typedef LoadingWidget = Widget Function(List);
typedef ProviderList = List Function(BuildContext);

class _DefaultLoadingWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: Center(
          child: CircularProgressIndicator(),
        ),
      ),
    );
  }
}

abstract class _AwaitProviders extends SingleChildStatelessWidget {
  final Widget child;
  final LoadingWidget loading;
  final ProviderList values;

  const _AwaitProviders({Key key, this.child, this.loading, this.values})
      : super(key: key);

  @override
  Widget buildWithChild(BuildContext context, Widget child) {
    final _values = values(context);
    if (_values.contains(null)) {
      if (loading != null) {
        return loading(_values);
      }
      return _DefaultLoadingWidget();
    }
    return child;
  }
}

class AwaitProvider<T> extends _AwaitProviders {
  AwaitProvider({Key key, Widget child, LoadingWidget loading})
      : super(
          key: key,
          child: child,
          loading: loading,
          values: (context) => [
            Provider.of<T>(context),
          ],
        );
}