lookup method Null safety
- Object? element
If an object equal to
object is in the set, return it.
object is in the set, like contains, and if so,
returns the object in the set, otherwise returns
If the equality relation used by the set is not identity,
then the returned object may not be identical to
Some set implementations may not be able to implement this method.
If the contains method is computed,
rather than being based on an actual object instance,
then there may not be a specific object instance representing the
E? lookup(Object? element) => _source.lookup(element);