Also:
- "rewrite" node Op in exporter for some nodes instead of importer
- more comments

Change-Id: I809e6754d14987b28f1da9379951ffa2e690c2a7
Reviewed-on: https://go-review.googlesource.com/22008Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>