This is a follow-up to a326c3e1 to avoid reflect being in the API.

Fixes #12801.

Change-Id: Ic4c2e592e2c35b5911f75d88f1d9c44787c80f30
Reviewed-on: https://go-review.googlesource.com/15240
Run-TryBot: David Symonds <dsymonds@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>