API could still be made more Go-ey.

Updates #15165.

Change-Id: I514ffceffa43c293ae5d7e5f1e9193fda0098865
Reviewed-on: https://go-review.googlesource.com/21644Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>