This makes use of an expando property (__reactInternalPrevContext) on the stateNode (instance). A property on the instance was used instead of a Fiber attribute because:
1) Conditional fields on fibers would break monomorphism.
2) Adding to all fibers would bloat types that don't use context.

This affects Stack and Fiber. I've updated impacted tests as well.
This may not be the direction we choose to go either. More discussion is still needed about the future of context...