DDC: Implement null spread element typing with null-safety

[eernstg]

Cf. dart-lang/language#1079, Never is the inferred element/key/value type of a spread element of the form ...?e where the static type of e is a potentially nullable subtype of Null, and Never is also the inferred element/key/value type of ...e where the static type of e is a subtype of Never.

DDC currently reports several errors for the associated tests: https://dart-review.googlesource.com/c/sdk/+/154693.

[eernstg]

Note, cf. #42760 (comment), that this issue might be resolved automatically when #42758 is resolved.

[sigmundch]

Closing since the fix fffd0d0 for the CFE addressed all the ddc failures as well.