Don't report `ReversedMissingMethodProblem` for sealed traits

[raboof]

If I understand correctly, we report ReversedMissingMethodProblem when checking for backwards compatibility because if a method was added to trait A, and trait B is compiled against an older version of A, trait B might be missing forwarders that a project that depends on both B and the newer A could run into.

If I understand that correctly, then we could suppress ReversedMissingMethodProblem for sealed traits, right?

[JD557]

I was thinking a bit about this and, if I understand the problem correctly, you need more guarantees than just "being a sealed trait" - It needs to be a sealed trait that is only extended by classes.

For example:

sealed trait Sealed

trait Unsealed extends Sealed

In this case, it´s possible for code outside of the file to extend Unsealed, which would pretty much break any bincompat guarantees that you get by having Sealed marked as sealed.

[plokhotnyuk]

The similar problem was reported when all traits are sealed.