[arxanas]

Neither Hack nor Flow are reified, so it's a different matter.

Both of these languages started out with strict null-checking — Hack because there were too many bizarre falsy values in PHP to allow otherwise, and Flow because it worked out so well in Hack. So neither had to tack on strict null-checking afterwards.

There have been some similar large-scale migration efforts. For example, Hack's record types were built on top of PHP arrays, which don't distinguish between absent and null values. Consequently, Hack's record types didn't distinguish between optional and nullable fields. Furthermore, records support width subtyping by default. This is unsound: you can construct distinct types A and B with A <: B <: A.

After a lot of effort, we did migrate the entire codebase to resolve this unsoundness. Essentially, we changed every record declaration to be an "open" record type and made all new records "closed" by default (referring to whether they supported width subtyping).

The analogue, then, would be something like refactoring every type declaration in every consumer's Dart code to be annotated as nullable, adding explicit null-checks, and allowing them to write non-nullable types as the default thenceforth.