To kill async.iterator or not to kill

[megawac]

As far as I can tell, async.iterator is not used internally despite being documented as an exposed internal method. As mentioned in #1220, I think it's time we leave this method in the dust in favour of ES2015 iterator protocol (https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Iterators_and_Generators) which natively supports arrays

[aearly]

Yeah, I think we can kill it. It used to be used internally in waterfall, and I think caolan just exposed it in case anyone found it useful. It's been superseded by native iterator support.