Map helpers with lists as values

map_list.dart

///This extension contains some usefull helpers for maps
///which value contains a list of [V] objects.
extension MapList<K, V> on Map<K, List<V>>
{

  ///Add [value] to the list of [key].
  ///If this map doesn't contain the [key],
  ///a new list is added with [value] as only value.
  void add(K key, V value)
  {
    if(containsKey(key) && this[key] is List<V>) {
      this[key]!.add(value);
    } else {
      this[key] = [value];
    }
  }

}