Method AppendInt

src/ffi/testing/testing.cc:750–753

  // Some methods to exercise RegisterMethod
  int64_t AddInt(int64_t x) const { return v_int + x; }
  Array<int64_t> AppendInt(Array<int64_t> xs, int64_t y) const {
    xs.push_back(y);
    return xs;
  }
  Optional<String> MaybeConcat(Optional<String> a, Optional<String> b) const {
    if (a.has_value() && b.has_value()) return String(a.value() + b.value());
    if (a.has_value()) return a;