setter generation for var in class generates wrong code

[aappddeevv]

https://gitter.im/lampepfl/dotty?at=5d88c6115d24375b3e075ca0

I'm claiming this generates the wrong code because it gives a warning under "-strict".

It appears that this warning pops up alot when -strict is set. I'm also getting these warnings on given instance declarations.

[smarter]

I think -strict should just not emit warnings for def foo_=, setters should be writable without using @alpha.