Running with --enable-experiment=non-nullable and an agnostic mode dill, we can successfully compile a trivial program to sound null safety. Unfortunately, this fails at runtime with:

[VERBOSE-2:shell.cc(209)] Dart Error: error: Library 'package:hello_world/main.dart' was compiled with null safety (in strong mode) and it requires --null-safety option at runtime

I tried passing this flag via --dart-flags, but that did not change the messaging.