num easeInElastic(num ratio)

Source 

static num easeInElastic(num ratio) {
  if (ratio == 0.0 || ratio == 1.0) return ratio;
  ratio = ratio - 1.0;
  return -pow(2.0, 10.0 * ratio) * sin((ratio - 0.3 / 4.0) * (2.0 * PI) / 0.3);
}