Makes compress/flate work better with cmd/dist bootstrap.

Change-Id: Ifc7d74027367008e82c1d14ec77141830583ba82
Reviewed-on: https://go-review.googlesource.com/111815
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>