lookup method
override
If an object equal to object
is in the set, return it.
Checks if there is an object in the set that is equal to object
.
If so, that object is returned, otherwise returns null.
Implementation
V lookup(Object element) {
if (element != null && element is! V) return null;
var key = _keyForValue(element as V);
return _baseMap[key];
}