Provide a clearer message if a user connects to a DartVM that isn't connected to Flutter Tools.

From a user on discord:

> Today, i tried something new...
> i ran an android app with a view in flutter (add-to-app) and observed that when the FlutterEngine is initialised, i get a log saying flutter: Observatory listening on http://127.0.0.1:38177//. I tried to open this in Flutter Devtools (by installing Termux, exposing this port to the internet using ngrok and opening this in Devtools) but got a message saying "This application does not have any tabs".
> What is different when the app is run using flutter run and when the debug apk is run? if there is no way to open the observatory, why the log?

The problem is the Dart VM likely cannot evaluate expressions as Flutter tools isn't attached to it. We see this combined with a VM that appears to be debug mode and get confused and show zero tabs instead of showing a message like "VM Service not fully initialized, run `flutter attach` so that you can debug.

Start by tracing the handling of the "This application does not have any tabs" message when a Dart VM is not fully connected to Flutter Tools. Confirm the VM-service initialization state and update the user-facing result to explain that `flutter attach` is required; done means this case no longer appears as an empty tab set.

索引モデルが issue の本文から書いたものです。