Null subtyping is inconsistent

[smarter]

I'm not sure what the semantics of Null should be exactly, but:

object O {
  // This compiles
  val a: { type T } = null;
  val b: Any { type T } = null;

  // This doesn't:
  // found   : Null
  // required: AnyRef{T}
  val c: AnyRef { type T } = null;
}

The problem is that the null subtyping check is done with:

(tp1.symbol eq NullClass) && tp2.dealias.typeSymbol.isNullableClass

But AnyRef is not a class (it's an alias of Object) so isNullableClass fails. What would be the most robust way to fix this?
By the way, should we remove NotNull? It's been deprecated in scalac already: scala/scala#2244
CC @odersky