Repository: flutter
Issue: [Web] Router initialRoute is breaking reporttoEngine and find assert from engine
Labels: framework,engine,f: routes,platform-web,has reproducible steps,P2,found in release: 2.0,found in release: 2.1,team-web,triaged-web

I have made an demo with Router for web.

1. i try to add following code to initial route in main.dart, run attach demo, you can find the reportsRouteUpdateToEngine is not working, the uri in web browser will not change, when i push new page.

``` dart
      routeInformationProvider: PlatformRouteInformationProvider(
        initialRouteInformation: const RouteInformation(
          location: '/home',
        ),
      ),
```

2. implement currentConfiguration in router_delegate.dart.

  /// If this getter returns non-null, the [Router] will start to report new
  /// route information back to the engine. In web applications, the new
  /// route information is used for populating browser history in order to
  /// support the forward and the backward buttons.

``` dart 
  @override
  String get currentConfiguration =>
      _pages.isNotEmpty ? _pages.last.name : null;

```
and if you push PageA to PageB, you will get following assert info. It's from https://github.com/flutter/engine/blob/410dd6ff3cc8b23828f4b0a88a65f73945e7eb0a/lib/web_ui/lib/src/engine/window.dart#L115

```
"Receives old navigator update in a router application. This can happen if you use non-router versions of MaterialApp/CupertinoApp/WidgetsApp together with the router versions of them."
    at Object.throw_ [as throw] (http://localhost:59703/dart_sdk.js:5032:11)
    at Object.assertFailed (http://localhost:59703/dart_sdk.js:4971:15)
    at _engine.EngineSingletonFlutterWindow.new.handleNavigationMessage (http://localhost:59703/dart_sdk.js:170524:32)
    at handleNavigationMessage.next (<anonymous>)
    at runBody (http://localhost:59703/dart_sdk.js:37229:34)
```
[router_initial_route_demo.zip](https://github.com/flutter/flutter/files/6075749/router_initial_route_demo.zip)

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      Studio.app/Contents/jre/jdk/Contents/Home/bin/java
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• No issues found!
```

</details>