void attachRootWidget(Widget rootWidget)

Takes a widget and attaches it to the renderViewElement, creating it if necessary.

This is called by runApp to configure the widget tree.

See also RenderObjectToWidgetAdapter.attachToRenderTree.

Source 

void attachRootWidget(Widget rootWidget) {
  _renderViewElement = new RenderObjectToWidgetAdapter<RenderBox>(
    container: renderView,
    debugShortDescription: '[root]',
    child: rootWidget
  ).attachToRenderTree(buildOwner, renderViewElement);
}