rotateAround(angle, vec) {
    let s = Math.sin(angle);
    let c = Math.cos(angle);
    this.x -= vec.x;
    this.y -= vec.y;
    let x = this.x * c - this.y * s;
    let y = this.x * s + this.y * c;
    this.x = x + vec.x;
    this.y = y + vec.y;
    return this;
  }
Rotates this vector around another vector.

@param {Number} angle The angle in radians to rotate the vector.
@param {Vector2} vec The vector which is used as the rotational center.
@returns {Vector2} Returns itself.