function createRect(a) {
  return a ? new GoogRect(a[0], a[1], a[2] - a[0], a[3] - a[1]) : null;
}
Create a GoogRect with given coordinates.
@param {Array<number>} a Array of numbers with given coordinates for rect,
    with expected order [x1, y1, x2, y2].
@return {?GoogRect} A rectangle, if coordinates were given.