anchor.diff:    ///
     /// This is an O(N) algorithm, and should not be necessary except for debug
     /// asserts. To avoid people depending on it, this function is implemented
  -  /// only in checked mode.
  +  /// only in debug mode, and always returns false in release mode.
     bool debugIsVisible(OverlayEntry entry) {
       bool result = false;
       assert(_entries.contains(entry));
  
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: /// [offstage] widgets which are kept alive, and are built, but are not laid out
  /// or painted.
  ///
  /// The onstage widget must be a [Stack].
  ///
  /// For convenience, it is legal to use [Positioned] widgets around the offstage
  /// widgets.
  