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: 
    void _remove(OverlayEntry entry) {
      if (mounted) {
        _entries.remove(entry);
        setState(() { /* entry was removed */ });
      }
    }
  
  
negative.code: 
    /// Insert the given entry into the overlay.
    ///
    /// If `above` is non-null, the entry is inserted just above `above`.
    /// Otherwise, the entry is inserted on top.
    void insert(OverlayEntry entry, { OverlayEntry above }) {
      assert(entry._overlay == null);
  