Flutter: testing, how to extrapolatepart of the UI

I have something like:

class _RootPageState extends State<RootPage> {

@override
  Widget build(BuildContext context) {
    return StreamBuilder(
      key: Key("rootStreamBuilder"),
        stream: _bloc.loadRootPage,
        builder: (context, snapshot) {
          if (snapshot.hasData) {
                  ...

        });

I want to test this StreamBuilder, how can I extrapulate from this class? So that I can do something like:

 testWidgets("test", (WidgetTester tester) async {


     await tester.pumpWidget(new MaterialApp(
       home: StreamBuilder
              ...