As specified in Annex B.3.6., initializers are allowed in sloppy mode var
declarations in for-in loops.

As a consequence, "in" is one of the tokens that can follow an
AssignmentExpression - this was not recognized before.

BUG=v8:6715

Change-Id: Idbf79bda39beef4e568d630e0b5e239a34397952
Reviewed-on: https://chromium-review.googlesource.com/620728Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47463}