anchor.diff:    /// route has been popped off.
     ///
     /// ```dart
  +  /// PageRouteBuilder(
  +  ///   pageBuilder: (BuildContext context,
  +  ///       Animation<double> animation,
  +  ///       Animation<double> secondaryAnimation,
  +  ///   ) {
  +  ///     return Scaffold(
  +  ///       appBar: AppBar(title: const Text('Hello')),
  +  ///       body: const Center(
  +  ///         child: Text('Hello World'),
  +  ///       ),
  +  ///     );
  +  ///   },
     ///   transitionsBuilder: (
     ///       BuildContext context,
     ///       Animation<double> animation,
  
positive.code:   ///       child: child, // child is the value returned by pageBuilder
    ///     );
    ///   },
    /// );
    /// ```
    ///
    /// We've used [PageRouteBuilder] to demonstrate the [buildTransitions] method
    /// here. The body of an override of the [buildTransitions] method would be
    /// defined in the same way.
    ///
    /// When the [Navigator] pushes a route on the top of its stack, the
    /// [secondaryAnimation] can be used to define how the route that was on
  
negative.code:   /// unsaved [Form] data if the user attempts to back out of the form. In that
    /// case, use the [Form.onWillPop] property to register the callback.
    ///
    /// {@tool snippet}
    /// To register a callback manually, look up the enclosing [ModalRoute] in a
    /// [State.didChangeDependencies] callback:
    ///
    /// ```dart
    /// abstract class _MyWidgetState extends State<MyWidget> {
    ///   ModalRoute<dynamic>? _route;
    ///
    ///   // ...
    ///
    ///   @override
    ///   void didChangeDependencies() {
    ///    super.didChangeDependencies();
    ///    _route?.removeScopedWillPopCallback(askTheUserIfTheyAreSure);
    ///    _route = ModalRoute.of(context);
    ///    _route?.addScopedWillPopCallback(askTheUserIfTheyAreSure);
    ///   }
    /// }
    /// ```
    /// {@end-tool}
    ///
    /// {@tool snippet}
    /// If you register a callback manually, be sure to remove the callback with
    /// [removeScopedWillPopCallback] by the time the widget has been disposed. A
    /// stateful widget can do this in its dispose method (continuing the previous
    /// example):
    ///
    /// ```dart
    /// abstract class _MyWidgetState2 extends State<MyWidget> {
    ///   ModalRoute<dynamic>? _route;
    ///
    ///   // ...
    ///
    ///   @override
    ///   void dispose() {
    ///     _route?.removeScopedWillPopCallback(askTheUserIfTheyAreSure);
    ///     _route = null;
    ///     super.dispose();
    ///   }
    /// }
    /// ```
    /// {@end-tool}
    ///
    /// See also:
    ///
  