This fixed a TODO from cec289ea by marking RewritableExpressions as
rewritten in AddArrowFunctionFormalParameters when decomposing
Assignments into pattern/initializer.

Also added a set_rewritten() helper method to RewritableExpression
to simplify callsites.

Change-Id: Ifa36c9fb6c79193cbbcb168eedf7f782dc73a77b
Reviewed-on: https://chromium-review.googlesource.com/622353Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47524}