Interface dependencies should be on Dart SDK (not Flutter SDK) where possible.

[wstrange]

This is in relation to #9526 and #8618

To implement an Admin SDK you only want to depend only on the Dart SDK (for example, to run on Cloud Run).

If you look at the Javascript / Node APIs (to use an example) - the same API for Firestore works on the client and on the server (the implementations differ, but much of the api surface area is common).

For things like Cloud Firestore, the current Dart interfaces such as Document, Collection, etc. - depend on the Flutter SDK - which means the interfaces can't be re-used to implement an Admin SDK. Splitting out the common APIs would be a good first step on the road to an Admin SDK, and eventually a pure Dart client implementation.

There are some examples of community developed Admin SDK functionality (firebase_dart on pub) - but looking at the code they look like they just copy / paste or reimplement some of the interfaces. Seems tedious, and drift is highly likely.