function CircleZone(x, y, radius) {
      var _this;
      _this = _Zone.call(this) || this;
      _this.x = x;
      _this.y = y;
      _this.radius = radius;
      _this.angle = 0;
      _this.center = {
        x: x,
        y: y
      };
      return _this;
    }
Creates a new CircleZone.
@param {number} x - The x-coordinate of the circle's center.
@param {number} y - The y-coordinate of the circle's center.
@param {number} [radius] - The radius of the circle.