cmd/goimports: flush to disk before diffing

Port of gofmt change at golang.org/cl/36750

Change-Id: Ie0ef6770be6361264ed45a9a9ece950aba24e852
Reviewed-on: https://go-review.googlesource.com/39073
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
1 file changed