build method
Builds the final widget for this display mode for this item. Sub-classes implement this to build the widget that is appropriate for the given display mode.
Implementation
@override
Widget build(BuildContext context, CommandBarItemDisplayMode displayMode) {
// First, build the widget for the wrappedItem in the given displayMode,
// as it is always passed to the callback
var w = wrappedItem.build(context, displayMode);
return builder(context, displayMode, w);
}