toSet method
As Iterable.toSet.
Note that the implementation is efficient: it returns a copy-on-write
wrapper around the data from this BuiltSet
. So, if no mutations are
made to the result, no copy is made.
This allows efficient use of APIs that ask for a mutable collection but don't actually mutate it.
Implementation
@override
Set<E> toSet() => CopyOnWriteSet<E>(_set, _setFactory);