/**
* Look up the value of [key], or add a new value if it isn‘t there.
*
* Returns the value associated to [key], if there is one.
* Otherwise calls [ifAbsent] to get a new value, associates [key] to
* that value, and then returns the new value.
*
* Map<String, int> scores = {‘Bob‘: 36};
* for (var key in [‘Bob‘, ‘Rohan‘, ‘Sophena‘]) {
* scores.putIfAbsent(key, () => key.length);
* }
* scores[‘Bob‘]; // 36
* scores[‘Rohan‘]; // 5
* scores[‘Sophena‘]; // 7
*
* Calling [ifAbsent] must not add or remove keys from the map.
*/
V putIfAbsent(K key, V ifAbsent());
