rotate(theta) {
    let l = this.getLeftVector();
    let r = this.getRightVector();
    let sinTheta = Math.sin(theta);
    let cosTheta = Math.cos(theta);
    let x = cosTheta * (r.x - l.x) - sinTheta * (r.y - l.y) + l.x;
    let y = sinTheta * (r.x - l.x) - cosTheta * (r.y - l.y) + l.y;
    this.setRightVector(x, y);
    return this;
  }
Rotate the line by a given value (in radians). The center of rotation is the left vector.

@param {Number} theta The angle (in radians) to rotate the line.
@returns {Line} This line.