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)