arrowHead(x, y, radius, facingAngle, sweptAngle) {
        let a1 = facingAngle + sweptAngle/2 + Math.PI;
        let a2 = facingAngle - sweptAngle/2 + Math.PI;
        this.polygon([
            x + Math.cos(facingAngle)*radius, y + Math.sin(facingAngle)*radius,
            x + Math.cos(a1)*radius, y + Math.sin(a1)*radius,
            x + Math.cos(a2)*radius, y + Math.sin(a2)*radius
        ]);
    }
@param {!number} x The x-position of the center of the arrow head.
@param {number} y The y-position of the center of the arrow head.
@param {number} radius The radius of the circle the arrow head is inscribed inside.
@param {number} facingAngle The direction the arrow head is pointing towards.
@param {number} sweptAngle The angle swept out by the back of the arrow head, relative to its center (not the
point at the front).