anchor.diff:  ///
   /// A bottom navigation bar is usually used in conjunction with a [Scaffold],
   /// where it is provided as the [Scaffold.bottomNavigationBar] argument.
  +///
  +/// The bottom navigation bar's [type] changes how its [items] are displayed.
  +/// If not specified it's automatically set to [BottomNavigationBarType.fixed]
  +/// when there are less than four items, [BottomNavigationBarType.shifting]
  +/// otherwise.
  +///
  +///  * [BottomNavigationBarType.fixed], the default when there are less than
  +///    four [items]. The selected item is rendered with [fixedColor] if it's
  +///    non-null, otherwise the theme's [ThemeData.primaryColor] is used. The
  +///    navigation bar's background color is the default [Material] background
  +///    color, [ThemeData.canvasColor] (essentially opaque white).
  +///  * [BottomNavigationBarType.shifting], the default when there are four
  +///    or more [items]. All items are rendered in white and the navigation bar's
  +///    background color is the same as the
  +///    [BottomNavigationBarItem.backgroundColor] of the selected item. In this
  +///    case it's assumed that each item will have a different background color
  +///    and that background color will contrast well with white.
   ///
   /// See also:
   ///
  
positive.code:   /// The color of the selected item when bottom navigation bar is
    /// [BottomNavigationBarType.fixed].
    ///
    /// If [fixedColor] is null, it will use the theme's primary color. The [fixedColor]
    /// field will be ignored if the [type] is not [BottomNavigationBarType.fixed].
    final Color fixedColor;
  
    /// The size of all of the [BottomNavigationBarItem] icons.
  
negative.code: }
  
  /// A material widget displayed at the bottom of an app for selecting among a
  /// small number of views, typically between three and five.
  ///
  /// The bottom navigation bar consists of multiple items in the form of
  /// text labels, icons, or both, laid out on top of a piece of material. It
  