Close instances of "dart.core.Sink" in appmetrica_push_platform_interface

[mit-73]

lib/src/appmetrica_push_platform_interface.dart:32:34

   ╷
32 │     final StreamController<String> onMessageController =
   │ ┌──────────────────────────────────^
33 │ │       StreamController<String>.broadcast();
   │ └──────────────────────────────────────────^
   ╵

lib/src/appmetrica_push_platform_interface.dart:36:34

   ╷
36 │     final StreamController<String> onMessageOpenedAppController =
   │ ┌──────────────────────────────────^
37 │ │       StreamController<String>.broadcast();
   │ └──────────────────────────────────────────^
   ╵

To reproduce make sure you are using the lints_core and run flutter analyze lib/src/appmetrica_push_platform_interface.dart