static MultiMap litmultimap(O... x) {
  new MultiMap mm;
  litmultimap_impl(mm, x);
  ret mm;
}

static void litmultimap_impl(MultiMap mm, O... x) {
  for (int i = 0; i < x.length-1; i += 2)
    if (x[i+1] != null)
      mm.putAll(x[i], (L) x[i+1]);
}

Began life as a copy of #1001695