问为什么我不能在Flutter中水平居中一列SizeTransition？

EN

class MyWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(),
      body: AnimatedBox(),
    );
  }
}

class AnimatedBox extends StatefulWidget {
  @override
  createState() => _AnimatedBoxState();
}

class _AnimatedBoxState extends State<AnimatedBox> with SingleTickerProviderStateMixin {
  AnimationController _controller;

  @override
  void initState() {
    super.initState();
    _controller = AnimationController(
      vsync: this,
      duration: Duration(milliseconds: 400),
    );
  }

  @override
  void dispose() {
    _controller.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Column(
      children: <Widget>[
        RaisedButton(
          child: Text('animate forward'),
          onPressed: () {_controller.forward();},
        ),
        RaisedButton(
          child: Text('animate reverse'),
          onPressed: () {_controller.reverse();},
        ),
        const SizedBox(height: 100.0,),
        SizeTransition(
          child: Container(
            width: 200.0,
            height: 200.0,
            color: Colors.blue,
          ),
          sizeFactor: CurvedAnimation(
            curve: Curves.fastOutSlowIn,
            parent: _controller,
          ),
        ),
      ],
    );
  }
}

return Container(
      width: double.infinity,
      child: Column(