Callers pass strings sliced as necessary instead of giving
an offset.

Fixes #16350

Change-Id: I7ba896f6ff09e0fd0094ca6c5af5d9a81622f15e
Reviewed-on: https://go-review.googlesource.com/27206
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>