promise.js
function IteratorStep ( iterator ) {
    var result = IteratorNext(iterator);
    return IteratorComplete(result) === true ? false : result;
  }
function IteratorStep ( iterator ) {
    var result = IteratorNext(iterator);
    return IteratorComplete(result) === true ? false : result;
  }
7.4.5 IteratorStep ( iterator )