For Dart, we chose the path of sound null safety. This involved a tradeoff. In a few cases, this made migrating to null safety a bit more expensive but results in a sound type system with complete trust in the non-null type annotations.
Developers must migrate their apps:
“While the migration to null-safety was certainly not easy for a large-scale codebase like the MyBMW app, Google’s tools gave us great assistance in the migration process. After having the migration completed we enjoy having a less error-prone codebase.”, Christian Schmid, BMW AG