There's no IndexShortStr func, refer to Index instead.

Change-Id: I6923e7ad3e910e4b5fb0c07d6339ddfec4111f4f
Reviewed-on: https://go-review.googlesource.com/c/go/+/170124
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>