Adjust function literal return type inference

[eernstg]

Issue #3148 showed that we need to adjust the rules about function literal return type inference. This PR changes the rules accordingly.

The basic idea is that the last step in the inference of an async function used to use the inferred return type of Future<flatten(S)> where S is taken from the context or from the returned expressions, but it should instead be Future<S>. Also, flatten then needs to be applied to the type obtained from the context, such that we still have the same return type as before when the context type gets to decide the outcome.

[eernstg]

We need to use futureValueType(R), not flatten(R). If we were to use flatten then we'd carry ? over from the context. So we could have a context type that wants a Future<num>?, then we'd get int? as the value of R, and then we'd get an inferred return type of Future<int?> where the context wants a return type of Future<int>?, and that's a type error.

Future<int>? Function() f = () async => 1;

In this example R is Future<int>? and flatten(R) is int?. But we don't want the function literal to have a return type of Future<int?>, it should be Future<int>.

[eernstg]

Closing: #3151 contains a PR which simplifies a larger number of locations in this section because it avoids wrapping K in FutureOr in the first place, and that's an overall improvement.