await expressions are an invalid destructuring target, and should
result in a SyntaxError when used in a position where a destructuring
target is expected.

BUG=v8:7173
R=marja@chromium.org, adamk@chromium.org

Change-Id: I1bdb4bc13cb2e3e904fc4389a6e0abca1e0ed17f
Reviewed-on: https://chromium-review.googlesource.com/811946Reviewed-by: Sathya Gunasekaran (ooo until 12/12) <gsathya@chromium.org>
Commit-Queue: Caitlin Potter <caitp@igalia.com>
Cr-Commit-Position: refs/heads/master@{#49977}