rotate(angle) {
    let tmp = new Vector2(0, 0);
    let cosAngle = Math.cos(angle);
    let sinAngle = Math.sin(angle);
    tmp.x = this.x * cosAngle - this.y * sinAngle;
    tmp.y = this.x * sinAngle + this.y * cosAngle;
    this.x = tmp.x;
    this.y = tmp.y;
    return this;
  }
Rotates this vector by a given number of radians around the origin of the coordinate system.

@param {Number} angle The angle in radians to rotate the vector.
@returns {Vector2} Returns itself.