Use case

As its part of the Flutter philosophy to allow breaking changes between version to keep the framework improving you immediately get the problem how to ensure that your app is build with a compatible version.
Adding the whole Flutter repository to each project seems sub-optimal.

Proposal

When defining a specific flutter version in pubspec flutter should automatically checkout/download the correct version and build the app.
Ideally in a separate folder so that after the build the latest selected version is active again.