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: /// and the corresponding size factor is null then the widget will match its
  /// child's size in that dimension. If a size factor is non-null then the
  /// corresponding dimension of this widget will be the product of the child's
  /// dimension and the size factor. For example if widthFactor is 0.5 then
  /// the width of this widget will always be half of the child's width.
  ///
  /// See also:
  ///
  ///  * [CustomSingleChildLayout]
  ///  * [Center] (which is the same as [Align] but with the [alignment] always
  ///    set to [FractionalOffset.center])
  ///  * [FractionallySizedBox] which sizes its child based on its own size.
  class Align extends SingleChildRenderObjectWidget {
    /// Creates an alignment widget.
    ///
  
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({
  