Conversion to Nodes still happens sequentially at the moment.

Change-Id: I3407ba0711b8b92e22ece0a06fefaff863c3ccc9
Reviewed-on: https://go-review.googlesource.com/35126
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>