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.

Source 

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