The bug was that we didn't track using await as a class name inside
arrow function formal parameters, and hence didn't recognize the error
in this case:

async(x = class await {}) => {}

BUG=v8:6714

Change-Id: Iabe6c947a4f621fb72361671d77f4765ba1a9578
Reviewed-on: https://chromium-review.googlesource.com/616776Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47407}