Implement error about awaiting a type that is incompatible with await

[eernstg]

The PR dart-lang/language#3560 introduces the notion that a type can be incompatible with await. This concept is used to specify a new and broader error about expressions of the form await e where the static type of e has associations with an extension type that does not implement Future.

The previous rules just made it an error when the type was such an extension type, now it is also an error when it is T? where T is incompatible with await, or a type variable that extends such a type, etc. Details in dart-lang/language#3560. Example:

extension type N(Future<int> _) {}
extension type F(Future<int> _) implements Future<int> {}

void test<X, XN extends N, XF extends F>(
    N n, F f, X x, XN xn, XF xf, N? nq, F? fq, XN? xnq, XF? xfq) async {
  await n; // Error.
  await f; // OK, type `int`.
  await x; // OK, type `X`.
  await xn; // Error.
  await xf; // OK, type `int`.
  await nq; // Error.
  await fq; // OK, type `int?`.
  await xnq; // Error.
  await xfq; // OK, type `int?`.
  if (x is N) await x; // Error.
  if (x is N?) await x; // Error.
  if (x is XN) await x; // Error.
  if (x is XN?) await x; // Error.
  if (x is F) await x; // OK, type `int`.
  if (x is F?) await x; // OK, type `int?`.
  if (x is XF) await x; // OK, type `int`.
  if (x is XF?) await x; // OK, type `int?`.
}

Subtasks:

• [analyzer] Implement error about awaiting a type that is incompatible with await #54648
• [cfe] Implement error about awaiting a type that is incompatible with await #54649

[eernstg]

Associated test: https://github.com/dart-lang/sdk/blob/main/tests/language/async/await_incompatible_test.dart.

[stereotype441]

Here's the current state of things:

• Specification: The initial specification for this error was made in Adjust the rule about await e where the type of e is an extension type language#3560 on Jan 17. The specification was updated in Adjust incompatible-with-await to be more strict on intersections language#3598 on Jan 31.
• CFE implementation: The CFE implemented the initial specification in 1672177 on Jan 29, and the update in a5bbbe8 on Feb 2.
• Analyzer implementation: the analyzer implemented the initial specification in adee0a2 on Feb 9. It has not implemented the update yet.

The branch cut for Dart 3.3 stable was made on Jan 3, so neither the CFE implementation nor the partial analyzer implementation has shipped in any stable release yet.

We (the language team) would like to cherry-pick this new error to a point release of Dart 3.3 stable as soon as possible, to help keep users of extension types from awaiting them in problematic ways. Although this new error is technically a breaking change, it concerns a minor corner case of a brand new language feature; we believe adoption of the new language feature is still low enough that if we act fast, it will be ok to make the cherry-pick without going through the breaking change process.

In order to do that, we need to first complete the analyzer implementation, then do a coordinated cherry-pick of 1672177, a5bbbe8, adee0a2, and the remaining analyzer work, so that they all land in the same point release of Dart 3.3 stable.

I will coordinate with the analyzer team to get the remaining analyzer work done, and then I'll file the cherry-pick request and coordinate with the scrum team to get it landed.

[stereotype441]

As of 690d49c, this is now fully supported in the main branch by the analyzer and front end.

[stereotype441]

The fix has been cherry-picked to the stable branch and should go out in the next hotfix release. See #55095 for cherry-pick details.