Co-hacking with josharian at Gophercon.

Change-Id: Ia59dfab676c6ed598c2c25483439cd1395a4ea87
Reviewed-on: https://go-review.googlesource.com/12029Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>