Latest stager is incompatible with latest Dart SDK

A roll of the Dart SDK into Flutter has revealed that devtools_app is incompatible with the latest Dart SDK.

devtools_app depends on [stager](https://github.com/bryanoltman/stager); the latest release of stager depends on a specific range of source_gen and of the analyzer package, both of which prohibit analyzer 8.x.y or 9.x.y.

The latest Dart SDK on the main channel includes dot-shorthand syntax in `dart:developer` sources. Any version of the analyzer package predating 8.4.1 cannot parse this code.

What to do?

* Bump stager - In June, I was able to land a PR in the stager GitHub repo that [widens the constraint on source_gen](https://github.com/bryanoltman/stager/commit/16c8bdc023df1c92211980124fc2956323aee8df). This code has not been published on pub, and the repo has been idle since that PR. This PR was actually for a different issue, and bumping the constraint to include source_gen ^3.0.0, and also to require analyzer ^8.0.0 or ^9.0.0, would be new work.
* Drop stager - Can we drop stager? Can we drop it temporarily so that devtools can continue to be tested in the Flutter customer tests? It seems to be mostly used in some tests, as it is mostly referenced in `test/test_infra/scenes`.

Commencez par inspecter les références à stager sous test/test_infra/scenes ainsi que les contraintes de dépendances de devtools_app. Comparez les contraintes disponibles de source-gen et analyzer de stager avec la dernière version du Dart SDK, puis déterminez s’il est viable de mettre à jour ou de supprimer stager. Le travail est terminé lorsque le projet prend en charge l’analyzer actuel et que les Flutter customer tests s’exécutent correctement.

Rédigé par le modèle d'indexation à partir du texte de l'issue.