Add'when' conditional statement

[tmxdyf]

Hope to add a conditional statement similar to the ‘when’ in the Kotlin language.

dart language:

  FutureOr<void> onEventHandler(
      SpeechPageEvent event, Emitter<SpeechPageState> emit) async {
    debugPrint("onEventHandler#event==$event");

    if (event is InitSpeechBlocEvent) {
      
    } else if (event is OnLongPressStartEvent) {
      
    } else if (event is OnLongPressEndEvent) {
      
    } else if (event is OnLongPressMoveUpdateEvent) {
      
    } else if (event is OnLeftLanguagePressedEvent) {
      
    } else if (event is OnRightLanguagePressedEvent) {
      
    } 
  }

Kotlin

when(event){
    is InitSpeechBlocEvent->{

    }
    is OnLongPressStartEvent->{

    }
    ...
}

[lrhn]

This sound like a job for pattern matching.

Also #1509, #27.