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()));
}
}
```

まず、appBar、bottomNavigationBar、および openList 内の Navigator.push 呼び出しを備えた WebviewScaffold を使ってサンプルを再現します。scaffold と push された route がどのように構成されているかを確認します。元の WebView が下に残ったままにならず、2 ページ目が表示されれば完了です。

索引モデルが issue の本文から書いたものです。