removeListener method
Implementation
void removeListener(String type, OnMessageCallback callback) {
if ((!_listeners.containsKey(type)) ||
(!_listeners[type]!.contains(callback))) return;
_listeners[type]!.remove(callback);
}
void removeListener(String type, OnMessageCallback callback) {
if ((!_listeners.containsKey(type)) ||
(!_listeners[type]!.contains(callback))) return;
_listeners[type]!.remove(callback);
}