For whatever reason, the SDK dill we are building right now is not in sound mode.

Test to confirm: https://github.com/dart-lang/sdk/blob/master/tests/language/nnbd/constant_null_safety_mode_test.dart

Correct rule: https://github.com/dart-lang/sdk/blob/0ce83987d65d008a5705edb248bddb9c845643ec/utils/compile_platform.gni#L10

Example usage: https://github.com/dart-lang/sdk/blob/master/utils/dartdevc/BUILD.gn#L278

Initially should just check if dill is sound, follow up can confirm SDK is sound.