Warn when casing CSS vendor prefixes incorrectly

Fixes #2487.

Test Plan: jest
Stop treating key={null} as an unspecified key

This reverts commit dd92786fb038c4eb7c6545928646ec8aa9ecfa36, which was intended to be temporary for the 0.12 release.

Fixes #2394.
Update stripTypes transform to use fixed up jstransform transform