void retainAll(Iterable<Object> elements)

Removes all elements of this set that are not elements in elements.

Checks for each element of elements whether there is an element in this set that is equal to it (according to this.contains), and if so, the equal element in this set is retained, and elements that are not equal to any element in elements are removed.

Source 

void retainAll(Iterable<Object> elements) {
  // Build a set with the same sense of equality as this set.
  SplayTreeSet<E> retainSet = new SplayTreeSet<E>(_comparator, _validKey);
  int modificationCount = _modificationCount;
  for (Object object in elements) {
    if (modificationCount != _modificationCount) {
      // The iterator should not have side effects.
      throw new ConcurrentModificationError(this);
    }
    // Equivalent to this.contains(object).
    if (_validKey(object) && _splay(object as dynamic/*=E*/) == 0) {
      retainSet.add(_root.key);
    }
  }
  // Take over the elements from the retained set, if it differs.
  if (retainSet._count != _count) {
    _root = retainSet._root;
    _count = retainSet._count;
    _modificationCount++;
  }
}