spawn_companion() has stdout existance as an invariant
spawn_companion() has stdout existance as an invariant

Summary:
The documentation shows that if `stdout` is a `PIPE` that this will not be `None`

> Standard output stream (StreamReader) or None if the process was created with stdout=None

So let's make it an invariant so we don't have to appease the typechecker

Reviewed By: c-ryan747

Differential Revision: D19833960

fbshipit-source-id: 26ac2693962f6cdfbaca3d7154fb64bb5ec0a6b2

facebook/FBSimulatorControl,facebook/FBSimulatorControl,facebook/FBSimulatorControl,facebook/FBSimulatorControl,facebook/FBSimulatorControl