This advances asyncIteratorP updating iterationObserver with each
successive value. The iterationObserver may only be interested in certain
occurrences (updateState, finish, fail), so for convenience,
observeIterator feature tests for those methods before calling them.
This advances
asyncIteratorPupdatingiterationObserverwith each successive value. TheiterationObservermay only be interested in certain occurrences (updateState,finish,fail), so for convenience,observeIteratorfeature tests for those methods before calling them.