This reverts commit 96698b55.

Reason for revert: This patch was correct when it landed, but later,
the spec was changed to V8's old behavior in
https://github.com/tc39/ecma262/pull/885 .

Original change's description:
> [parser] allow ASI when "await" or "yield" follows "let"
> 
> Per https://github.com/tc39/test262/pull/956, André believes that ASI
> should be permitted in these situations.
> 
> BUG=
> R=​marja@chromium.org, adamk@chromium.org, littledan@chromium.org
> 
> Change-Id: I5602d8a507576607750ffa9e873e1bfa53dd3523
> Reviewed-on: https://chromium-review.googlesource.com/472568
> Reviewed-by: Marja Hölttä <marja@chromium.org>
> Commit-Queue: Caitlin Potter <caitp@igalia.com>
> Cr-Commit-Position: refs/heads/master@{#44585}

TBR=adamk@chromium.org,marja@chromium.org,littledan@chromium.org,caitp@igalia.com

# Not skipping CQ checks because original CL landed > 1 day ago.

Change-Id: I2c5bf709867da539ccd4cd82f3be98c8a0301f31
Reviewed-on: https://chromium-review.googlesource.com/553617Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Daniel Ehrenberg <littledan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46317}