This enables the format test to process this file (the format
test doesn't handle indexed formats, and this is the only place
in the compiler where they occur).

Change-Id: I99743f20c463f181a589b210365f70162227d4e0
Reviewed-on: https://go-review.googlesource.com/28932
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>