title: `vsync` property in TabController constructor
body: <p>According to this: <a href="https://docs.flutter.io/flutter/material/TabController-class.html" rel="noreferrer">sample code</a></p>
  
  <p>I created my own implementation of TabController:</p>
  
  <pre><code>void main() {
    runApp(new MyApp());
  }
  
  class MyApp extends StatefulWidget {
  
    @override
    _MyAppState createState() =&gt; new _MyAppState();
  }
  
  class _MyAppState extends State&lt;MyApp&gt; {
  
    TabController _tabController;
  
    @override
    void initState() {
      super.initState();
      _tabController = new TabController(vsync: this, length: choices.length);
    }
  
    @override
    void dispose() {
      _tabController.dispose();
      super.dispose();
    }
  
    @override
    Widget build(BuildContext context) {
      return new MaterialApp(
        home: new Scaffold(
          bottomNavigationBar: new Material(
            color: Colors.blue,
            child: new TabBar(
              controller: _tabController,
              isScrollable: false,
              tabs: choices.map((Choice choice) {
                return new Tab(
                  text: null,
                  icon: new Icon(choice.icon),
                );
              }).toList(),
            ),
          ),
          appBar: new AppBar(
            title: const Text('Swap'),
          ),
          body: new TabBarView(
            controller: _tabController,
            children: choices.map((Choice choice) {
              return new Padding(
                padding: const EdgeInsets.all(16.0),
                child: new ChoiceCard(choice: choice),
              );
            }).toList(),
          ),
        ),
      );
    }
  }
  </code></pre>
  
  <p>In line: <code>_tabController = new TabController(vsync: this, length: choices.length);</code> I got error this message:</p>
  
  <blockquote>
    <p>error: The argument type '_MyAppState' can't be assigned to the parameter type 'TickerProvider'. (argument_type_not_assignable at [swap] lib/main.dart:24)</p>
  </blockquote>
  
  <p>What is wrong with my code?</p>