operator []= method
- K key,
 - V value
 
override
    Associates the key with the given value.
If the key was already in the map, its associated value is changed. Otherwise the key/value pair is added to the map.
Implementation
@override
void operator []=(K key, V value) {
  if (super.containsKey(key)) {
    _value[key] = value;
    _changes.add(MapChangeNotification<K, V>.updated(key, key, value));
  } else {
    _value[key] = value;
    _changes.add(MapChangeNotification<K, V>.added(key, value));
  }
  refresh();
}