evaluate whether it is worth switching from build-runner to frontend-server

As mentioned in https://github.com/dart-lang/sdk/issues/50664#issuecomment-1347089380 we may want to consider whether it makes sense for webdev to switch to use a front-end-server solution for all uses, and not just for flutter.

There are many open questions to consider:
* analyze the tradeoffs between the two
* how to support sourcegen?
* how to centralize frontend server and expose it from the Dart SDK

Start with the linked Dart SDK discussion and inspect how webdev currently uses build-runner. Compare the tradeoffs of a frontend-server approach, including sourcegen support and exposing the server from the Dart SDK. Done means a documented recommendation that addresses the listed open questions.

Written by the indexing model from the issue text.