The rule for packages in this directory is that they can depend on nothing but core Dart packages. They can‘t depend on dart:ui, they can’t depend on any package:, and they can't depend on anything outside this directory.
Currently they do depend on dart:ui, but only for VoidCallback and clampDouble (and maybe one day lerpDouble), which are all intended to be moved out of dart:ui and into dart:core.
There is currently also an unfortunate dependency on the platform dispatcher logic (SingletonFlutterWindow, Brightness, PlatformDispatcher, window), though that should probably move to the ‘services’ library.
See also: