retainWhere method

void retainWhere (bool test(E element))
override

Removes all elements of this set that fail to satisfy test.

Implementation 

void retainWhere(bool test(E element)) {
  List toRemove = [];
  for (E element in this) {
    if (!test(element)) toRemove.add(element);
  }
  removeAll(toRemove);
}