static <A, C extends Collection<A>> C assertDoesntContain(C c, A y) {
  ret assertDoesntContain(null, c, y);
}

static <A, C extends Collection<A>> C assertDoesntContain(S msg, C c, A y) {
  if (contains(c, y))
    fail((msg != null ? msg + ": " : "") + y + " contained in " + c);
  ret c;
}

Began life as a copy of #1012660