[analyzer] Implement new rules about 'derives a future type'

[eernstg]

The language team has just agreed to land dart-lang/language#3574, which is an update to the definition of the function flatten and the associated concept about how "a type T derives a future type F".

Some background discussion can be found in #54628. The main point is that the new definition includes the ability to find a supertype of the form Future<...>, FutureOr<...>, Future<...>?, or FutureOr<...>? that were previously not found.

This change is not considered to be a breaking change because the CFE already implements the updated rule.

A test of the new rule is landed by https://dart-review.googlesource.com/c/sdk/+/354020, as tests/language/async/derive_future_type_test.dart.

[bwilkerson]

@scheglov

[eernstg]

I can see that the language specification at https://spec.dart.dev/DartLangSpecDraft.pdf hasn't been updated along with the latest update to the LaTeX source.

The reason for this is that the automatic generation of a PDF from the source failed, because of an unrelated failure (shown on the Dart CI log at https://github.com/dart-lang/language/actions/runs/8084567444/job/22725574044).

I reported the issue here: dart-lang/language#3656.