Original code fixed in https://go-review.googlesource.com/#/c/36359/.

Fixes #18820.

Change-Id: I060e6c9d0e312b4fd5d0674aff131055bf5cf61d
Reviewed-on: https://go-review.googlesource.com/36412
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Adam Langley <agl@golang.org>
Reviewed-on: https://go-review.googlesource.com/36414Reviewed-by: Austin Clements <austin@google.com>