When --harmony-async-iteration is enabled, it is now possible to
use the for-await-of loop, which uses the Async Iteration protocol
rather than the ordinary ES6 Iteration protocol.

the Async-from-Sync Iterator object is not implemented in this CL,
and so for-await-of loops will abort execution if the iterated object
does not have a Symbol.asyncIterator() method. Async-from-Sync
Iterators are implemented seperately in https://codereview.chromium.org/2645313003/

BUG=v8:5855, v8:4483
R=neis@chromium.org, littledan@chromium.org, adamk@chromium.org

Review-Url: https://codereview.chromium.org/2637403008
Cr-Commit-Position: refs/heads/master@{#43224}