[cfe] Account for overall nullability of FutureOrType in UP
[cfe] Account for overall nullability of FutureOrType in UP

Closes https://github.com/dart-lang/sdk/issues/48631

Change-Id: I944da3a7f086a3d080de4f51bd99aa002f88cae6
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/239024
Reviewed-by: Johnni Winther <0f64ef784d57ffb064b634040f9eeba0d33d0be4@google.com>
Commit-Queue: Chloe Stefantsova <0b401402caf5616842b565557744ed97c1e5b2a2@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk