distanceSq(vec) {
    return (vec.x - this.x) * (vec.x - this.x) + (vec.y - this.y) * (vec.y - this.y);
  }
Returns the squared euclidean distance between this vector and another vector. When only the relative distances of a set of vectors are needed, this is is less expensive than using distance(vec).

@param {Vector2} vec Another vector.
@returns {Number} The squared euclidean distance of the two vectors.