Within `class.together` subclasses cannot access parent fields and methods without `this`

[samdphillips]

Outside of class.togther the B class can access the parent's attributes without this in the foo method.

class.together:
  class A(s :~ String):
    nonfinal

    property t: "this is t"

    method u():
      "uuuu"

  class B():
    extends A

    method foo():
      // s
      // t
      u()

/*
  u: unbound identifier
    in: u
*/