forEach method
Applies action
to each key-value pair of the map in order of MRU to
LRU.
Calling action
must not add or remove keys from the map.
Implementation
@override
void forEach(void Function(K key, V value) action) {
var head = _head;
while (head != null) {
action(head.key, head.value);
head = head.next;
}
}