As you noticed, its more like fromIteratorFactory. It would basically be:

abstract class Iterator<T> {  
  // ...
  Iterator.fromIteratorFactory(Iterator<T> Function() makeIterator) = _IterableFromIteratorFunction<T>;
  // ...
}

final class _IterableFromIteratorFactory<T> extends Iterable<T> {
  final Iterator<T> Function() _makeIterator;
  IterableFromIteratorFactory(this._makeIterator);
  Iterator<T> get iterator => _makeIterator();
}

Not unreasonable.

The reason I would very rarely use such a class is that I'd want to optimize some of the Iterable operations. If I can't, then there's rarely a reason to have an iterable at all.
Other people's use may differ.

Do you have a concrete example you can link to?

If sync* had equivalent performance would you prefer it over the separate Iterator class implementation?

Would iterable generator expressions solve your use case? dart-lang/language#1633

I do think this is worth adding. I hope we can find a shorter name than fromIteratorFactory.

Hey @natebosch, @lrhn, I'm working on this, please assign this issue to me, I've submitted an initial pr #59908, there we can discuss if any changes are needed or if any additional test cases I need to cover

Code looks fine at a first glance.
I agree with @natebosch that the name is too long.

Maybe just fromIterables would be enough to signify that it's not just one Iterable.
Or withIterable where the singular word refers to the Iterable.iterator that the argument is giving an implementation of.

(I wish Dart had anonymous classes like Java, so you could just do

new Iterable{get iterator {…}}

rather than having to have all these "from functions" constructors.)

Yeah, I couldn't find a better name for this

I think Iterable.withIterator makes sense and signifies the use case, anything else before I rename and push the changes?