Based on Filippo Valsorda's https://golang.org/cl/24230

Fixes #16094

Change-Id: Ie39b0834e220f0a0f4fbfb3bbb271e70837718c3
Reviewed-on: https://go-review.googlesource.com/32478Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>