Comparison of certain map types could fail to be antisymmetric.
This corrects that.

Change-Id: I88c6256053ce29950ced4ba4d538e241ee8591fe
Reviewed-on: https://go-review.googlesource.com/22552
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: jcd . <jcd@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>