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];
}