The flutter/core-packages repo is now live, but it doesn't have any CI yet. I'm filing this to track the subset of #185027 that's related to getting CI working:

• Add recipe support, similar to flutter/packages (a single generic recipe), but relying only on Dart, not Flutter.
• Make whatever Cocoon changes are necessary to include a new repo
• Add a .ci.yaml file and get things running
• Add a version of the flutter/packages release GitHub Action workflow, which will require having the publishing fcontrib account's secret in available.