addCommandListener method
Implementation
Future<void> addCommandListener(void callback(Command<String> response),
{void onError(String error)?}) =>
_methodChannel.invokeMethod(
"Backendless.Messaging.Channel.addCommandListener", {
"channelHandle": _channelHandle
}).then((handle) => BackendlessMessaging._commandCallbacks[handle] =
new EventCallback(callback, onError));