This generates the same code as before, but does so directly rather
than building an AST and printing that. This is in preparation for
later changes.

Change-Id: Ifec141120bcc74847f0bff8d3d47306bfe69b454
Reviewed-on: https://go-review.googlesource.com/c/142883
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>