anchor.diff:  
     /// The dimensions of the rectangle into which the application will be drawn,
     /// in physical pixels.
  +  ///
  +  /// When this changes, [onMetricsChanged] is called.
  +  ///
  +  /// At startup, the size of the application window may not be know before Dart
  +  /// code runs. If this value is observed early in the application lifecycle,
  +  /// it may report [Size.zero].
  +  ///
  +  /// This value does not take into account any on-screen keyboards or other
  +  /// system UI. The [padding] and [viewInsets] properties provide a view into
  +  /// how much of each side of the application may be obscured by system UI.
     ///
     /// See also:
     ///
  
positive.code:   /// when the device is rotated or when the application is resized (e.g. when
    /// showing applications side-by-side on Android).
    ///
    /// The framework invokes this callback in the same zone in which the
    /// callback was set.
    VoidCallback get onMetricsChanged => _onMetricsChanged;
    VoidCallback _onMetricsChanged;
    Zone _onMetricsChangedZone;
  
negative.code:   /// The Flutter framework operates in logical pixels, so it is rarely
    /// necessary to directly deal with this property.
    ///
    /// When this changes, [onMetricsChanged] is called.
    ///
    /// See also:
    ///
    ///  * [WidgetsBindingObserver], for a mechanism at the widgets layer to
  