This makes these names even less likely to collide with a real user-defined function.

Fixes #13852.

Change-Id: If5a8562c6797ced19c355c7ab2c86fc4401a8674
Reviewed-on: https://go-review.googlesource.com/21490
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>