anchor.diff:  }
   
   /// Metadata for identifying children in a [CustomMultiChildLayout].
  +/// The [MultiChildLayoutDelegate] hasChild, layoutChild, and positionChild
  +/// methods use these identifiers.
   class LayoutId extends ParentDataWidget<CustomMultiChildLayout> {
     /// Marks a child with a layout identifier.
     ///
  
positive.code:   /// incoming width constraint multipled by this factor.
    final double widthFactor;
  
    /// If non-null, the fraction of the incoming height to give to the child.
    ///
    /// If non-null, the child is given a tight height constraint that is the max
    /// incoming height constraint multipled by this factor.
  
negative.code:   }
  }
  
  /// Sizes its child to a fraction of the total available space.
  /// For more details about the layout algorithm, see [RenderFractionallySizedOverflowBox].
  ///
  /// See also:
  /// * [Align] (which sizes itself based on its child's size and positions
  ///   the child according to a [FractionalOffset] value)
  /// * [OverflowBox]
  class FractionallySizedBox extends SingleChildRenderObjectWidget {
    FractionallySizedBox({
  