auto<K, V> static method
Returns a map that will be created only when necessary, such as when Map.putIfAbsent is called.
It assumes the map was empty, and creates by invoking creator
when necessary. Don't use this method if the map already exists.
It is useful to save the memory, if you have a map that won't contain anything in most case.
Notice you don't have to keep the object being returned by this method,
since it is just a proxy to the real map.
Refer to Rikulo UI's View.dataset for a sample implementation.
Implementation
static Map<K, V> auto<K, V>(Map<K, V> creator()) => _OnDemandMap(creator);