The widget subtree that will "fly" from one route to another during a Navigator push or pop transition.
The appearance of this subtree should be similar to the appearance of the subtrees of any other heroes in the application with the same tag. Changes in scale and aspect ratio work well in hero animations, changes in layout or composition do not.
This widget can only have one child. To lay out multiple children, let this
widget's child be a widget such as Row, Column, or Stack, which have a
children property, and then provide the children to that widget.
final Widget child