static threePointangle(vecA, vecB, vecC) {
    let ab = Vector2.subtract(vecB, vecA);
    let bc = Vector2.subtract(vecC, vecB);
    let abLength = vecA.distance(vecB);
    let bcLength = vecB.distance(vecC);
    return Math.acos(Vector2.dot(ab, bc) / (abLength * bcLength));
  }
Returns the angle between two vectors based on a third vector in between.

@static
@param {Vector2} vecA A vector.
@param {Vector2} vecB A (central) vector.
@param {Vector2} vecC A vector.
@returns {Number} The angle in radians.