fillPolygon(vertices, fillColor) {
        let last = vertices[vertices.length - 1];

        this.ctx.beginPath();
        this.ctx.moveTo(last.x, last.y);
        for (let p of vertices) {
            this.ctx.lineTo(p.x, p.y);
        }

        this.ctx.fillStyle = fillColor;
        this.ctx.fill();
    }
Draws the inside of a polygon.
@param {!(!Point[])} vertices
@param {!string} fillColor