Load local webpage from assets

I try to load a local webpage from my assets:

```dart
new WebviewScaffold(
url: "file:///assets/web/loesungen.html",
allowFileURLs: true,
…
```
But if I open the view in Android emulator I get the error message:

The webpage as **file:///assets/web/loesungen.html** could not be loaded because:
net::ERR_FILE_NOT_FOUND

The assets webpage is correct included in pubspec.yaml and I can load the webpage with:

```dart
final htmlData = await rootBundle.loadString('assets/web/loesungen.html');
```
I user Flutter 1.2.1, Dart 2.1.2 and flutter_webview_plugin 0.3.0

Any ideas why it’s not work?

Reproduce the failure from the WebviewScaffold example on the Android emulator, comparing the file:///assets/web/loesungen.html URL with the rootBundle.loadString('assets/web/loesungen.html') entry point. Read the asset declaration in pubspec.yaml and the plugin's Android WebView handling; done means identifying whether this asset URL is supported and documenting or correcting the loading path.

索引モデルが issue の本文から書いたものです。