[ddc] hide unsound flag if build requires it

[sigmundch]

Starting in Dart3.beta, the SDK will have a new build flag to determine whether an SDK supports unsound null safety on any of its commands. When the flag indicates that unsound mode is not supported, then DDC needs to provide an error if a user provides the --no-sound-null-safety flag.

[nshahan]

Is there an issue for adding the build flag that we can list here as blocking for now? Or if that has already landed, how can we tell if DDC is running a version that was built with it enabled?

[a-siva]

I have created #50348 for the VM which would have the build rule changes etc. Would this be the issue that you are referring to above ?

[nshahan]

I'm not sure. If I'm understanding correctly it seems like the DDC binary script will need to know if it is running as part of a Dart SDK that was built with a special build flag enabled. My expectation is that flag would be outside of the VM if it is passed as part of the SDK build.

[itsjustkevin]

@nshahan are we planning this work still as we have changed how we are approaching the no-sound-null-safety flag?

[sigmundch]

@nshahan - since we no longer include the unsound .dill files (since https://dart-review.googlesource.com/c/sdk/+/278999), is it fair to mark this one as complete?

[nshahan]

I'm closing the issue because at the moment I'm not planning any more work here but with the caveat that there is still work in progress in the Flutter repo.

Running DDC is only supported via running builds with the Flutter Tool or build_web_compilers.

• Flutter Tools is close to removing support for --no-sound-null-safety. It landed but needed to be reverted.
  Remove option to pass unsound null safety flag in flutter build/run commands flutter/flutter#118810

• build_web_compilers has removed support for `` in the version that works with Dart 3.0
  Drop support for unsound null safety in Dart 3 build#3424

The unsound .dill files used by DDC have been removed from the packaged SDK.