Takes 3% off my all.bash run time.

For #10571.

Change-Id: I8f00f523d6919e87182d35722a669b0b96b8218b
Reviewed-on: https://go-review.googlesource.com/18087Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>