In Dart 3 only sound null safety will be allowed.

The dart tool already prints a warning in the compile command when using unsound or no null safety, but we should update to provide more details, and to link to the dart.dev null safety page for more details.

We discussed also adding a warning message to the run command, but decided against that as adding even Info messages to run can be breaking due to scripts and command line app tests looking at the console output.