putIfAbsent method
Look up the value of key, or add a new entry if it isn't there.
If the key/value pair is new or changed then a MapUpdate is sent to attached streams.
Implementation
@override
V? putIfAbsent(K key, V? Function() ifAbsent) {
final currentV = this[key];
if (currentV != null) {
return currentV;
}
final newV = ifAbsent();
this[key] = newV;
return newV;
}