imports: return when done channel is closed

Change-Id: I49317a947ec58572fd16078279678d4514cc2a4b
Reviewed-on: https://go-review.googlesource.com/37919
Run-TryBot: Matt Layher <mdlayher@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matt Layher <mdlayher@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
1 file changed