[dart2js] default to sound null safe mode

[sigmundch]

Today dart2js autodetects the mode based on the entrypoint.

• Milestone 1: Starting Dart 3.alpha, dart2js should compile in sound mode by default and provide an error if the application contains files not migrated to null safety.
• Milestone 2: In Dart 3.beta, the unsound flag will be exposed selectively. It will be available if an SDK build supports it (this will be determined by an SDK build flag that doesn't exist yet, potentially could be something like --enable-sound-mode)

[mit-mit]

@sigmundch is there anything remaining for alpha?

[sigmundch]

Alpha changes are complete, thanks!

[itsjustkevin]

@sigmundch with the approach we are taking with the flag, is this work still necessary for the release?

[sigmundch]

We are still planning to make this change.

We default to sound null safety in dart2js, but the option of using --no-sound-null-safety is still available. We would like to make that an error. @nshahan has a change in progress to make that happen.

[nshahan]

Update:

• Change has been prepared and reviewed https://dart-review.googlesource.com/c/sdk/+/289280.
• Just waiting for the corresponding fix to land in the golem repo first.

[sigmundch]

Nick's changes landed with 7c0e4d6, this is now complete