attachRootWidget method
Takes a widget and attaches it to the renderViewElement, creating it if necessary.
This is called by runApp to configure the widget tree.
Implementation
void attachRootWidget(Widget rootWidget) {
_renderViewElement = RenderObjectToWidgetAdapter<RenderBox>(
container: renderView,
debugShortDescription: '[root]',
child: rootWidget
).attachToRenderTree(buildOwner, renderViewElement);
}