whichSide(vecA, vecB) {
    return (this.x - vecA.x) * (vecB.y - vecA.y) - (this.y - vecA.y) * (vecB.x - vecA.x);
  }
Calculates which side of a line spanned by two vectors this vector is.

@param {Vector2} vecA A vector.
@param {Vector2} vecB A vector.
@returns {Number} A number indicating the side of this vector, given a line spanned by two other vectors.