In order to upstream app.dart we need to reimplement WebOnlyNavigatorObserver to use platform messages instead relying on isWeb flag.

1. https://github.com/flutter/flutter_web/blob/master/packages/flutter_web/lib/src/material/app.dart
2. https://github.com/flutter/flutter_web/blob/master/packages/flutter_web/lib/src/widgets/web_navigator.dart