if you add a new 9999 package w/out a non-9999 ebuild, the uprev stage emits a warning like:
09:29:01: WARNING: Missing stable ebuild in /b/c/cbuild/repository/src/private-overlays/chromeos-overlay/sys-kernel/chromeos-kernel-4_4

have we outlived the usefulness of this ?  seems like we should change it to an info:
INFO: No stable ebuild found (will create a new one during uprev) in .../src/private-overlays/chromeos-overlay/sys-kernel/chromeos-kernel-4_4

as a general thing, i can see the argument for the warning (based on this func living in lib.portage_util in common metadata code), but in practice, the uprev logic is the only thing that uses it.