[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)

[sigmundch]

FYI - re dart2js - @natebiggs has a CL in progress to update the dart2js invocation to use sound null safety. He is out this week, but we can get a more accurate estimate when he returns next week.

[a-siva]

This CL https://dart-review.googlesource.com/c/sdk/+/283981 attempts to add a build time option for removing the unsound null safety flag, currently this fails to build the SDK, once @natebiggs lands the CL to update dart2js I can try building the SDK again.

[biggs0125]

I expect to land my CL (https://dart-review.git.corp.google.com/c/sdk/+/280221) early next week. I'm waiting for the equivalent internal change to roll around a bit to make sure nothing comes up.

[biggs0125]

I've landed the above change, Dart2JS should now run in sound null safe mode.

[a-siva]

I've landed the above change, Dart2JS should now run in sound null safe mode.

Confirmed that the Dart SDK builds successfully now.

[a-siva]

Here is what I learnt about the use of Flutter's Dart SDK internally from @chingjun
Flutter's Dart SDK is used in the following scenarios

• to compile a kernel / AOT that needs to be compatible with a prebuilt Flutter Engine (Dart VM)
• to run a prebuilt dill file

Basically all Flutter Dart code internally will be compiled with the frontend_server that is in the downloaded Dart SDK
The --no-sound-null-safety flag is being passed when calling the kernel_worker and frontend_server as all this code has not been migrated.

This pretty much means we cannot remove the -no-sound-null-safety flag from the 'dart compile kernel' or dart compile aot-snapshot commands in dart cli until all this code is migrated. The Dart VM will have to retain the ability to run unsound null safe code when it reads a dill file that was compiled with that option.

[a-siva]

We have decided to not implement this build flag for 3.0 and hence removing the milestone, the option of locking down the no-sound-null-safety option in the compiler has already landed.