update lint and fix analyzer comments

Author: lohanidamodar

templates/flutter/lib/package.dart.twig

@@ -2,15 +2,12 @@ library {{ language.params.packageName }};
 
 import 'dart:async';
 import 'dart:typed_data';
-import 'package:flutter/foundation.dart';
-import 'package:http/http.dart' show MultipartFile;
 import 'src/enums.dart';
 import 'src/client.dart';
 import 'src/service.dart';
 import 'src/input_file.dart';
 import 'models.dart' as models;
 import 'src/upload_progress.dart';
-import 'src/exception.dart';
 
 export 'src/response.dart';
 export 'src/client.dart';

templates/flutter/pubspec.yaml.twig

@@ -19,6 +19,6 @@ dependencies:
   web_socket_channel: ^2.2.0
 
 dev_dependencies:
-  flutter_lints: ^1.0.4
+  flutter_lints: ^2.0.1
   flutter_test:
     sdk: flutter