BMFDot constructor
- {@required BMFCoordinate center,
 - @required double radius,
 - @required Color color,
 - int zIndex: 0,
 - bool visible: true}
 
BMFDot构造方法
Implementation
BMFDot(
    {@required this.center,
    @required this.radius,
    @required this.color,
    int zIndex: 0,
    bool visible: true})
    : super(zIndex: zIndex, visible: visible);