cast<R> method
Makes this iterable useful as an Iterable<R>, if necessary.
Like retype except that this iterable is returned as-is
if it is already an Iterable<R>.
It means that someIterable.cast<Object>().toList() is not guaranteed
to return precisely a List<Object>, but it may return a subtype.
Implementation
Queue<R> cast<R>() {
Queue<Object> self = this;
return self is Queue<R> ? self : Queue.castFrom<E, R>(this);
}