anchor.diff:  /// [offstage] widgets which are kept alive, and are built, but are not laid out
   /// or painted.
   ///
  -/// The onstage widget must be a Stack.
  +/// The onstage widget must be a [Stack].
   ///
   /// For convenience, it is legal to use [Positioned] widgets around the offstage
   /// widgets.
  
positive.code: 
    /// The state from the closest instance of this class that encloses the given context.
    ///
    /// In checked mode, if the [debugRequiredFor] argument is provided then this
    /// function will assert that an overlay was found and will throw an exception
    /// if not. The exception attempts to explain that the calling [Widget] (the
    /// one given by the [debugRequiredFor] argument) needs an [Overlay] to be
    /// present to function.
    ///
    /// Typical usage is as follows:
  
negative.code: 
    void _remove(OverlayEntry entry) {
      if (mounted) {
        setState(() {
          _entries.remove(entry);
        });
      }
    }
  
  