test for https://github.com/flutter/flutter/issues/30568

tab_bar_test.dart

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: TabBarTest(title: 'Test Tab Bar'),
    );
  }
}

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

  final String title;

  @override
  _TabBarTestState createState() => _TabBarTestState();
}

class _TabBarTestState extends State<TabBarTest> with TickerProviderStateMixin {
  TabController _tabController;
  List<Widget> _tabs = [
    Container(decoration: BoxDecoration(color: Colors.red)),
    Container(decoration: BoxDecoration(color: Colors.blue)),
    Container(decoration: BoxDecoration(color: Colors.greenAccent)),
  ];

  @override
  void initState() {
    _tabController = TabController(vsync: this, length: 3);
    super.initState();
  }

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

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text('tabBarTesting')),
      body: TabBarView(children: _tabs, controller: _tabController),
      bottomNavigationBar: TabBar(controller: _tabController, tabs: const <Tab>[
        Tab(icon: Icon(Icons.person), text: 'Home'),
        Tab(icon: Icon(Icons.school), text: 'Grades'),
        Tab(icon: Icon(Icons.settings), text: 'Settings')
      ]),
    );
  }
}