Actually execute topological sort to get those special dependencies right.

Mistake introduced in CL 67650.

Change-Id: I22fd6efb4f033deaf7f191431c0401b59b8a97d0
Reviewed-on: https://go-review.googlesource.com/67870
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>