prop not present when added by a global mixin

Version

3.0.0-rc.8

Reproduction link

https://jsfiddle.net/f1v72cm5/6/

Steps to reproduce

See jsfiddle link.

What is expected?

Vue 2 behavior:
https://jsfiddle.net/0umc5ojf/2/

What is actually happening?

Vue warn: Property was accessed during render but is not defined on instance...