Also add MustClose and MustWriter to cmd/internal/bio, and use them in
cmd/asm.

Change-Id: I07f5df3b66c17bc5b2e6ec9c4357d9b653e354e0
Reviewed-on: https://go-review.googlesource.com/21938
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>