Expose a smaller surface area for custom watchers (dart-lang/watcher#92)

- Remove the function to unregister a custom watcher, we can add it
  later when we have a specific use case for it.
- Remove the class `CustomFactoryWatcher` and take callbacks instead.
- Bump the version and add the changelog that was missed.
- Add missing copyright notice.
4 files changed
tree: 4db908454d86a41b4ab953c764582ef40b9b6d35
  1. pkgs/