Breaking change 54828: Make the type schema for null-aware spread operations consistent

[Paul Berry]

The type schema used by the compiler front end to perform type inference on the operand of a null-aware spread operator (...?) in map and set literals will be made nullable, to match the behavior of list literals (and to match the behavior of the analyzer).

Although this is technically a breaking change, it's not expected to have any effect on real-world code.

More details can be found here: https://github.com/dart-lang/sdk/issues/54828

If you have any concerns about this change, please leave comments in the linked issue.

Thank you!
Paul

[Paul Berry]

This change was implemented in https://github.com/dart-lang/sdk/commit/c9e2b012b1132a4fc10ee9a8793651f6b1c807ba.