Function flatArray

docs/tests/v2/es6/js/sanddance.js:124998–125009

(points, fx, fy, that)

};

function flatArray(points, fx, fy, that) {
  const n = points.length;
  const array = new Float64Array(n * 2);

  for (let i = 0; i < n; ++i) {
    const p = points[i];
    array[i * 2] = fx.call(that, p, i, points);
    array[i * 2 + 1] = fy.call(that, p, i, points);
  }

  return array;
}

function* flatIterable(points, fx, fy, that) {
  let i = 0;