args[i] automatically returns undefined if i < 0 || i >= args.Length().

R=mstarzinger@chromium.org, titzer@chromium.org

Change-Id: I215545fa54ce23440f1de49b48786e568831bc82
Reviewed-on: https://chromium-review.googlesource.com/704586Reviewed-by: Ben Titzer <titzer@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48379}