circle(x, y, radius) {
        this.ctx.moveTo(x + radius, y);
        this.ctx.arc(x, y, radius, 0, 2 * Math.PI, false);
    }
@param {!number} x The x-coordinate of the center of the circle.
@param {!number} y The y-coordinate of the center of the circle.
@param {!number} radius The distance from the center of the circle to its side.