This is so the values can not be changed and the type is easy to see.

Requested on https://go-review.googlesource.com/c/go/+/120561.

Change-Id: If2ed48ca3ba8874074687bfb2375d2f5592e8e0d
Reviewed-on: https://go-review.googlesource.com/120564Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>