Address Russ's feedback from https://golang.org/cl/33244

TBR=See https://golang.org/cl/33244

Updates #17929

Change-Id: I708d71f519f6414ecec629d3c273d9e737d8ed50
Reviewed-on: https://go-review.googlesource.com/33656Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>