HashSet.from(Iterable elements)

Create a hash set containing all elements.

Creates a hash set as by new HashSet<E>() and adds each element of elements to this set in the order they are iterated.

All the elements should be assignable to E. The elements iterable itself may have any element type, so this constructor can be used to down-cast a Set, for example as:

Set<SuperType> superSet = ...;
Set<SubType> subSet =
    new HashSet<SubType>.from(superSet.where((e) => e is SubType));

Source 

factory HashSet.from(Iterable elements) {
  HashSet<E> result = new HashSet<E>();
  for (final e in elements) {
    E element = e as Object/*=E*/;
    result.add(element);
  }
  return result;
}