Second screen below WebviewScaffold

Using WebviewScaffold with appBar and bottomNavigationBar bugs the Navigation.push.

I'm using the WebviewScaffold as my primary route "/", with two IconButtons inside a Row, in the bottomNavigationBar. Also, using the AppBar inside the WebviewScaffold.
Opening the second page replaces only the AppBar and the BottomNavigationBar, the WebView is still there.

Maybe I'm doing something wrong with the Navigator, I don't know. :/

```dart
MaterialApp(
title: 'Flutter Demo',
debugShowCheckedModeBanner: false,
theme: ThemeData(
primarySwatch: Colors.blue,
),
routes: {
"/": (_) => WebView(),
},
);

...

class _WebViewState extends State {

@override
Widget build(BuildContext context) {
return new WebviewScaffold(
url: "https://www.google.com",
appBar: AppBar(title: Text("First WebPage")),
bottomNavigationBar: bottomNavigation(context),
);
}

@override
void dispose() {
_flutterWebView.dispose();
super.dispose();
}

/// Buttons
Row bottomNavigation(context) {
return Row(
children: [
IconButton(
icon: Icon(Icons.list),
onPressed: () {
openList(context);
}),
IconButton(icon: Icon(Icons.phone_android), onPressed: openNativeScreen)
],
mainAxisAlignment: MainAxisAlignment.center,
);
}

///Actions
void openList(BuildContext context) {
Navigator.push(context, new NavigationRoute(builder: (context) => new ListScreen()));
}
}
```

Commencez par reproduire l’exemple avec WebviewScaffold, son appBar, sa bottomNavigationBar et l’appel à Navigator.push dans openList. Examinez comment le scaffold et la route ouverte sont composés ; c’est terminé lorsque la deuxième page s’affiche sans que la WebView d’origine reste en dessous.

Rédigé par le modèle d'indexation à partir du texte de l'issue.