LayoutId constructor

LayoutId({Key key, @required Object id, @required Widget child })

Marks a child with a layout identifier.

Both the child and the id arguments must not be null.

Implementation 

LayoutId({
  Key key,
  @required this.id,
  @required Widget child
}) : assert(child != null),
     assert(id != null),
     super(key: key ?? ValueKey<Object>(id), child: child);