Revert a tiny recent refactoring in SelectiveCPSTransform

The refactoring in 7e6c723df means that we can't build the CPS
plugin if we skip locker in development mode.

This commit backs out the refactoring and leaves a TODO comment
to perform it at a later date.
Merge pull request #2932 from retronym/topic/cps-skip-locker-friendly

Revert a tiny recent refactoring in SelectiveCPSTransform