The rule for packages in this directory is that they can depend onnothing but core Dart packages. They can't depend on `dart:ui`, theycan't depend on any `package:`, and they can't depend on anythingoutside this directory.
Currently they do depend on dart:ui, but only for `VoidCallback` and`clampDouble` (and maybe one day `lerpDouble`), which are all intendedto be moved out of `dart:ui` and into `dart:core`.
There is currently also an unfortunate dependency on the platformdispatcher logic (SingletonFlutterWindow, Brightness,PlatformDispatcher, window), though that should probably move to the'services' library.
