关于 Dart 的重定向构造函数的疑问

上代码

class Point extends Test {
  num x;
  num y;
  num z;
  num get a {
    return this.x + this.y;
  }

  set a(num v) {
    x = v - y;
  }

  Point.fromJson(Map jsonMap)
      : this.x = jsonMap['x'],
        y = jsonMap['y'],
        super.fromJson({}) {
    print(this.x);
    print('In Point.fromJson(): ($x, $y, $z)');
  }

  Point.aliasB(Map jsonMap, num z)
      : this.z = z,
        super.fromJson({}) {
    Point.fromJson(jsonMap);
  }

  Point(Map jsonMap, this.z) : super(jsonMap) {
    print('In Point(): ($x, $y, $z)');
    x = jsonMap['x'];
    y = jsonMap['y'];
    print('In Point(): ($x, $y, $z)');
  }
}

class Test {
  num x;
  num y;
  Test(Map map)
      : x = map['x'],
        y = map['y'] {
    print('In Parent.Test(): ($x, $y)');
  }
  Test.fromJson(Map map) : this(map);
}

main(List<String> args) {
  new Point({'x': 1, 'y': 2}, 3);
  new Test({'x': 1, 'y': 2});
}

执行结果

In Parent.Test(): (null, null)
In Point(): (null, null, 3)
In Point(): (1, 2, 3)
In Parent.Test(): (1, 2)

疑问：

1：Test 构造函数冒号右边不是赋值了吗 为啥是 null ？接受的参数打印出来是有值的

2：如果我把 Test 类的成员变量 x,y 改名字不跟子类一样，就能打印出来，这又是为啥？

希望有大佬能解答一下菜鸟的疑问