Make sure we don't depend on map iterator order.

Fixes #14600

Change-Id: Iac0e0c8689f3ace7a4dc8e2127e2fd3c8545bd29
Reviewed-on: https://go-review.googlesource.com/20158
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>