anchor.diff:      ui.ImageFilterEngineLayer? oldLayer,
     }) {
       assert(filter != null); // ignore: unnecessary_null_comparison
  -    return _pushSurface(PersistedImageFilter(oldLayer as PersistedImageFilter?, filter)) as ui.ImageFilterEngineLayer;
  +    return _pushSurface<PersistedImageFilter>(PersistedImageFilter(oldLayer as PersistedImageFilter?, filter));
     }
   
     /// Pushes a backdrop filter operation onto the operation stack.
  
positive.code:     ui.PhysicalShapeEngineLayer? oldLayer,
    }) {
      assert(color != null, 'color must not be null'); // ignore: unnecessary_null_comparison
      return _pushSurface(PersistedPhysicalShape(
        oldLayer as PersistedPhysicalShape?,
        path as SurfacePath,
        elevation,
        color.value,
        shadowColor?.value ?? 0xFF000000,
        clipBehavior,
      )) as ui.PhysicalShapeEngineLayer;
    }
  
    /// Add a retained engine layer subtree from previous frames.
  
negative.code:   /// The surface currently being built.
    PersistedContainerSurface get _currentSurface => _surfaceStack.last;
  
    T _pushSurface<T extends PersistedContainerSurface>(T surface) {
      // Only attempt to update if the update is requested and the surface is in
      // the live tree.
      if (surface.oldLayer != null) {
  