Asynchronous iterators

New productions: TBD.

AST format change: TBD.

References:

• The proposal: https://github.com/tc39/proposal-async-iteration
• Change to the spec:
• ESTree change:

As of Feb 2017, it's still in Stage 3 (per https://github.com/tc39/proposals).

I think we can close this in favor of: https://github.com/jquery/esprima/issues/1990