customer_testing breaks every on every release branch. For example of a breakage #56450.

The engine repo has clone_flutter.sh, but we should have a general solution that supports multiple repos, multiple CI platforms (Cirrus/LUCI). It should be testable, and thus not in BASH. Dart would be the obvious choice, although then we have to solve how to depend on Dart in the engine builds.

I know that we have stable Dart in the devicelab environment. If our infra scripts rely on stable Dart, this would mean we don't have to build the Dart SDK from source in every shard, we could just rely on a CIPD package.

Things to clone:

• Customer testing (framework - cirrus)
• Flutter Gallery v2 (devicelab) Pin version of flutter/gallery in tests #62983
• Dart & Format (engine)