New asSeenFrom does not skolemize some types that should be skolemized

[smarter]

#999 changed the way asSeenFrom works and it introduced a bug:

class Contra[-T]

class Foo {
  val foo: Contra[this.type] = new Contra[this.type]
}
object Test {
  def test: Unit = {
    val e1 = new Foo
    val f1: Contra[Foo] = e1.foo // error, as expected
    var e2 = new Foo
    val f2: Contra[Foo] = e2.foo // should be an error but now succeeds
  }
}

The problem is that we removed this line:

val m = if (isLegalPrefix(pre)) null else new AsSeenFromMap(pre, cls)

but asSeenFrom#toPrefix still assumes that the prefix is always legal when the map is null.