[CP] Cherrypick CFE FutureOr nullability fixes to Dart 2.17

[Markzipan]

commit(s) to merge: 7d8d3bb

merge instructions: clean merge

What is the issue: Undetermined nullability for FutureOr types was leaking through the CFE into our backends (detected first in DDC), as reported by Flutter users.

What is the fix: CFE-side fixes in type substitution.

Why cherrypick: This is causing problems right now for some Flutter web users (e.g., consumers of https://pub.dev/packages/riverpod).

Risk: Little

Link to original issue(s): #48631

/cc @mit-mit @whesse @athomas @vsmenon @devoncarew @sigmundch @nshahan

[sigmundch]

I'm in favor of including this as part of 2.17 - the change is fairly small (most of the CL are tests, only 10 LOC of actual change), and it addresses an issue that is being hit by flutter web users.

@johnniwinther @leafpetersen - what do you think?

[johnniwinther]

LGTM

[athomas]

Merged to beta in e5fa9de (2.17.0-266.7.beta).