CFE: 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.

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