You cannot create a List from an Iterator<E> - construction requires an Iterable<E> which wraps an iterator getter that returns an Iterator<E>
 
It would be convenient if the programmer already has an iterator to use that rather than require the Iterator be wrapped in an Iteratable. For example if a function only requires an Iterator<E> without caring about what collection it came from (map, set, random generator, whatever) and internally wanted to represent that as a list while processing it.

Though to be fair it looks like List.from(...) just goes over the collection at least once, perhaps going over and allocating a second list if fixed length is requested. Ideally this cost would be hinted at in the documentation.

// Copied from list.dart
  /**
   * Creates a list and initializes it using the contents of [other].
   *
   * The [Iterator] of [other] provides the order of the objects.
   *
   * This constructor returns a growable list if [growable] is true;
   * otherwise, it returns a fixed-length list.
   */
  factory List.from(Iterable other, { bool growable: true }) {
    List<E> list = new List<E>();
    for (E e in other) {
      list.add(e);
    }
    if (growable) return list;
    return makeListFixedLength(list);
  }