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()));
}
}
```

Comienza reproduciendo el ejemplo con WebviewScaffold, su appBar, bottomNavigationBar y la llamada a Navigator.push en openList. Inspecciona cómo se componen el scaffold y la ruta a la que se navega; se considera terminado cuando la segunda página se muestra sin que la WebView original permanezca debajo.

Escrito por el modelo de indexación a partir del texto del issue.