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`hashValues` (and maybe one day `hashList` and `lerpDouble`), whichare all intended to be moved out of `dart:ui` and into `dart:core`.See also: * https://github.com/dart-lang/sdk/issues/27791 (`VoidCallback`) * https://github.com/dart-lang/sdk/issues/25217 (`hashValues`, `hashList`, and `lerpDouble`)