Also refactor common position filling code into a function.

Fixes #18011

Change-Id: I76528626da67a7309193fa92af1e361c8e2fcf84
Reviewed-on: https://go-review.googlesource.com/33631
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>