Manual port of CL 37598 (submitted for Go 1.9) to Go 1.8.1.

Fixes #19133.
Fixes #19151.

Change-Id: I51707ea35068a393022f554b391ee2638dba16b5
Reviewed-on: https://go-review.googlesource.com/39617
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>