anchor.diff:  
   /// Defines the layout and behavior of a [BottomNavigationBar].
   ///
  +/// For a sample on how to use these, please see [BottomNavigationBar].
   /// See also:
   ///
   ///  * [BottomNavigationBar]
  
positive.code: /// {@tool dartpad --template=stateful_widget_material}
  /// This example shows a [BottomNavigationBar] as it is used within a [Scaffold]
  /// widget. The [BottomNavigationBar] has three [BottomNavigationBarItem]
  /// widgets and the [currentIndex] is set to index 0. The selected item is
  /// amber. The `_onItemTapped` function changes the selected item's index
  /// and displays a corresponding message in the center of the [Scaffold].
  ///
  /// ![A scaffold with a bottom navigation bar containing three bottom navigation
  /// bar items. The first one is selected.](https://flutter.github.io/assets-for-api-docs/assets/material/bottom_navigation_bar.png)
  ///
  /// ```dart
  /// int _selectedIndex = 0;
  
negative.code: /// ```
  /// {@end-tool}
  ///
  /// {@tool dartpad --template=stateful_widget_material}
  /// This example shows a [BottomNavigationBar] as it is used within a [Scaffold]
  /// widget. The [BottomNavigationBar] has four [BottomNavigationBarItem]
  /// widgets, which means it defaults to [BottomNavigationBarType.shifting], and
  /// the [currentIndex] is set to index 0. The selected item is amber in color.
  /// With each [BottomNavigationBarItem] widget, backgroundColor property is
  /// also defined, which changes the background color of [BottomNavigationBar],
  /// when that item is selected. The `_onItemTapped` function changes the
  /// selected item's index and displays a corresponding message in the center of
  /// the [Scaffold].
  ///
  ///
  /// ```dart
  /// int _selectedIndex = 0;
  /// static const TextStyle optionStyle = TextStyle(fontSize: 30, fontWeight: FontWeight.bold);
  /// static const List<Widget> _widgetOptions = <Widget>[
  ///   Text(
  ///     'Index 0: Home',
  ///     style: optionStyle,
  ///   ),
  ///   Text(
  ///     'Index 1: Business',
  ///     style: optionStyle,
  ///   ),
  ///   Text(
  ///     'Index 2: School',
  ///     style: optionStyle,
  ///   ),
  ///   Text(
  ///     'Index 3: Settings',
  ///     style: optionStyle,
  ///   ),
  /// ];
  ///
  /// void _onItemTapped(int index) {
  ///   setState(() {
  ///     _selectedIndex = index;
  ///   });
  /// }
  ///
  /// @override
  /// Widget build(BuildContext context) {
  ///   return Scaffold(
  ///     appBar: AppBar(
  ///       title: const Text('BottomNavigationBar Sample'),
  ///     ),
  ///     body: Center(
  ///       child: _widgetOptions.elementAt(_selectedIndex),
  ///     ),
  ///     bottomNavigationBar: BottomNavigationBar(
  ///       items: const <BottomNavigationBarItem>[
  ///         BottomNavigationBarItem(
  ///           icon: Icon(Icons.home),
  ///           label: 'Home',
  ///           backgroundColor: Colors.red,
  ///         ),
  ///         BottomNavigationBarItem(
  ///           icon: Icon(Icons.business),
  ///           label: 'Business',
  ///           backgroundColor: Colors.green,
  ///         ),
  ///         BottomNavigationBarItem(
  ///           icon: Icon(Icons.school),
  ///           label: 'School',
  ///           backgroundColor: Colors.purple,
  ///         ),
  ///         BottomNavigationBarItem(
  ///           icon: Icon(Icons.settings),
  ///           label: 'Settings',
  ///           backgroundColor: Colors.pink,
  ///         ),
  ///       ],
  ///       currentIndex: _selectedIndex,
  ///       selectedItemColor: Colors.amber[800],
  ///       onTap: _onItemTapped,
  ///     ),
  ///   );
  /// }
  /// ```
  /// {@end-tool}
  /// See also:
  ///
  ///  * [BottomNavigationBarItem]
  