HashSet.from(Iterable elements)

Create a hash set containing all elements.

Creates a hash set as by new HashSet<E>() and adds all given elements to the set. The elements are added in order. If elements contains two entries that are equal, but not identical, then the first one is the one in the resulting set.

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;
}