The slash is replaced with os.PathSeparator before returning.
Split, SplitList are the exceptions; comments for them mention this.

Fixes golang/go#10122.

Change-Id: I66dbee8d09f378582e046be8df309a3930151820
Reviewed-on: https://go-review.googlesource.com/7310Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Reviewed-by: Rob Pike <r@golang.org>