[VM] Hide unsound null safety flag if build requires it

[a-siva]

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 the VM needs to provide an error if a user passes in the --no-sound-null-safety flag.

[a-siva]

This CL is blocked on the following

• Flip the test configurations in the Dart VM build bots to test strong null safety mode in all the configurations #50091
• migration of dart2js to sound null safety

[mit-mit]

Hi @a-siva what remains that blocks this change?

[a-siva]

Hi @a-siva what remains that blocks this change?

• dart2js fully migrated and the option removed from invocation of dart2js in the SDK builds
• Flutter tools changed to not pass this option in, see Remove option to pass unsound null safety flag in flutter build/run commands flutter/flutter#118810
• A path for how to deal with our test bots which still pass the option for legacy mode testing (one option considered was that our bots would still build the SDK with the option available in the legacy test configurations)