Previously we only created synthetic variables in the parser and not
in the preparser, causing mismatch in the preparsed scope data.

This patch creates the variables in both parsers.

Bug: v8:5367
Change-Id: I9c511d0b9212bd36816956b06dc204b0b5920e1c
Reviewed-on: https://chromium-review.googlesource.com/789848
Commit-Queue: Sathya Gunasekaran <gsathya@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49637}