问如何在BottomNavigationBarItem上对齐小部件？

EN

import 'package:flutter/material.dart';

void main() => runApp(new MyApp());

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'Flutter Demo',
      theme: new ThemeData(
        primarySwatch: Colors.blue,
        canvasColor: Colors.blue
      ),
      home: new MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);


  final String title;

  @override
  _MyHomePageState createState() => new _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {

  ValueNotifier<int> bottomNavNotifier = new ValueNotifier(0);

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(
        title: new Text(widget.title),
      ),
      bottomNavigationBar: new Column(
        mainAxisSize: MainAxisSize.min,
        children: <Widget>[
          new BottomNavHighlight(bottomNavNotifier),
          new BottomNavigationBar(
            type: BottomNavigationBarType.fixed,
            fixedColor: Colors.white,
            items: <BottomNavigationBarItem>[
              new BottomNavigationBarItem(icon: new Icon(Icons.create), title: new Text("Create")),
              new BottomNavigationBarItem(icon: new Icon(Icons.create), title: new Text("Create")),
              new BottomNavigationBarItem(icon: new Icon(Icons.create), title: new Text("Create")),
              new BottomNavigationBarItem(icon: new Icon(Icons.create), title: new Text("Create"))
            ],
            onTap: (int index){
              print(index);
              bottomNavNotifier.value = index;
            },
          )
        ],
      ),
      backgroundColor: Colors.white,// This trailing comma makes auto-formatting nicer for build methods.
    );
  }
}

class BottomNavHighlight extends StatefulWidget{

  final ValueNotifier<int> activeIndex;

  BottomNavHighlight(this.activeIndex);

  @override
  State createState() {
    return new _BottomNavHighlightState();
  }
}

class _BottomNavHighlightState extends State<BottomNavHighlight>{



  @override
  Widget build(BuildContext context) {
    List<Widget> items = <Widget>[
      new Expanded(child: new Container()),
      new Expanded(child: new Container()),
      new Expanded(child: new Container()),
    ];
    items.insert(
      widget.activeIndex.value,
      new Expanded(child: new Container(child: new Icon(Icons.play_circle_outline, size: 40.0,))),);
    return new Row(
      children: items,
    );
  }

  @override
  void initState() {
    super.initState();
    widget.activeIndex.addListener((){
      setState(() {
      });
    });
  }

  @override
  void dispose() {
    super.dispose();
    widget.activeIndex.dispose();
  }
}

import 'package:flutter/material.dart';
class Home extends StatefulWidget {
  @override
  _HomeState createState() => _HomeState();
}

class _HomeState extends State<Home> with SingleTickerProviderStateMixin {
  TabController tabController;
  int _index = 0;

  List<String> tabString = [
    'Discover',
    'Geners',
    'Artists'
  ];

  String _title = 'Discover';

  @override
  void initState() {
    super.initState();

    tabController = TabController(
      length: 3,
      vsync: this
    );
    this._index = 0;
    setState(() {
      this._title = tabString.first;
    });
  }

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

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(_title),
        actions: <Widget>[
          IconButton(
            icon: Icon(Icons.search),
            onPressed: (){},
          )
        ],
      ),
      body: new Container(
        color: const Color(0xffEEEEEE),
              child: TabBarView(
          children: <Widget>[
            Container(child:Text('1')),
            Container(child:Text('2')),
            Container(child:Text('3')),
          ],
          controller: tabController,
        ),
      ),

      bottomNavigationBar: BottomNavigationBar(
        type: BottomNavigationBarType.fixed,
        currentIndex: _index,
          onTap: (int _index) {
            setState(() {
              this._title = tabString[_index];
              this._index = _index;
              this.tabController.animateTo(_index);
            });
          },
          items: <BottomNavigationBarItem>[
            new BottomNavigationBarItem(
              icon: new Icon(Icons.dashboard),
              title: new Text("Discover"),
            ),
            new BottomNavigationBarItem(
              icon: new Icon(Icons.surround_sound),
              title: new Text("Geners"),
            ),
            new BottomNavigationBarItem(
              icon: new Icon(Icons.account_circle),
              title: new Text("Artists"),
            ),

          ]),
    );
  }
}