This allows us to stop tracking variables_ in the preparser.

This currently makes us track slightly more variables than neccessary in the
case `for (var ...` since `var ... of` needs to check conflicts with out simple
catch variables. We should probably track the names through a ScopedPtrList
instead of a ZonePtrList anyway. Then it won't matter anymore.

Change-Id: I64e3f9ab13af8269456439cf15b0bc4d5b9e5380
Reviewed-on: https://chromium-review.googlesource.com/c/1421360Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58960}