GitLab

 - Remove obsolete check and tests from js_checker.py
 - Convert Downloads page to use let/const as a proof of concept that it works.

Bug: 671426
Change-Id: I6ef56337c02e93b7389788ded0eaec00954457e5
Reviewed-on: https://chromium-review.googlesource.com/570901Reviewed-by: Michael Giuffrida <michaelpg@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488828}