Add async iterators. (#2885)
Add async iterators. (#2885)

* Add async iterators.

https://www.chromestatus.com/features/5727385018171392

* Async iterations are implemented in Fx57 (bug 1352312)

mdn/browser-compat-data,chrisdavidmills/browser-compat-data,mdn/browser-compat-data,chrisdavidmills/browser-compat-data