anchor.diff:  
     /// Enables this route to veto attempts by the user to dismiss it.
     ///
  +  /// {@tool snippet}
     /// This callback is typically added using a [WillPopScope] widget. That
     /// widget finds the enclosing [ModalRoute] and uses this function to register
     /// this callback:
  
positive.code:   /// topmost route, e.g. because the use pressed the back button, the
    /// primary animation runs from 1.0 to 0.0.
    ///
    /// The following example uses the primary animation to drive a
    /// [SlideTransition] that translates the top of the new route vertically
    /// from the bottom of the screen when it is pushed on the Navigator's
    /// stack. When the route is popped the SlideTransition translates the
    /// route from the top of the screen back to the bottom.
    ///
    /// ```dart
    /// PageRouteBuilder(
    ///   pageBuilder: (BuildContext context,
    ///       Animation<double> animation,
    ///       Animation<double> secondaryAnimation,
    ///       Widget child,
    ///   ) {
    ///     return Scaffold(
    ///       appBar: AppBar(title: Text('Hello')),
    ///       body: Center(
    ///         child: Text('Hello World'),
    ///       ),
    ///     );
  
negative.code:   ///
    /// Returns null if the given context is not associated with a modal route.
    ///
    /// {@tool snippet}
    ///
    /// Typical usage is as follows:
    ///
    /// ```dart
    /// ModalRoute<int>? route = ModalRoute.of<int>(context);
    /// ```
    /// {@end-tool}
    ///
    /// The given [BuildContext] will be rebuilt if the state of the route changes
    /// while it is visible (specifically, if [isCurrent] or [canPop] change value).
  