The Typer put the wrong type on String#index and String#lastIndexOf
builtins, with an off by one on the upper bound.

Bug: chromium:762874
Change-Id: Ia4c29bc2e8e1c85b6a7ae0b99f8aaabf839a5932
Reviewed-on: https://chromium-review.googlesource.com/660000Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47942}