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 hashValues (and maybe one day hashList and lerpDouble), which are all intended to be moved out of dart:ui and into dart:core.

See also:

• dart-lang/sdk#27791 (VoidCallback)
• dart-lang/sdk#25217 (hashValues, hashList, and lerpDouble)