Set.from(Iterable elements)

Creates a Set that contains all elements.

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

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

The created Set is a LinkedHashSet. As such, it considers elements that are equal (using ==) to be indistinguishable, and requires them to have a compatible Object.hashCode implementation.

The set is equivalent to one created by new LinkedHashSet<E>.from(elements).

Source 

factory Set.from(Iterable elements) = LinkedHashSet<E>.from;