[analyzer] Normalize inherited getter and setter types before comparing

[Yusufihsangorgel]

The check compared the combined getter and setter types with ==, which
is structural. The notion of "same type" used elsewhere in the language
normalizes FutureOr, so NORM(FutureOr<Object>) is Object and a
getter typed Object agrees with a setter typed FutureOr<Object>.
dynamic and Object? normalize to themselves and keep reporting.

This is the analyzer half of what @eernstg described in the issue, on
the example @scheglov asked about there.

Two tests. Object against FutureOr<Object> was red before this change
and is green after; dynamic against Object? pins the behavior that
must not move.

The CFE half is still open. inferFieldType in members_node.dart
compares the same two types with kernel ==, so it reports on the example
today and the two disagree until it changes as well. Happy to follow it
into the front end if that is the order you want.

Bug: #63635
TEST=pkg/analyzer/test/src/diagnostics/different_inherited_getter_and_setter_types_test.dart

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[eernstg]

Hi @Yusufihsangorgel, thanks for contributing a substantial amount of work!

However, as I wrote here, we must approach this topic very cautiously because it changes existing behavior at a rather fundamental level of the language. The language team will have to clarify the situation first, and we might need to refine and clarify the rules in this area. So I'm afraid this PR should not be landed at this time.

I hope you don't think we're wasting your time by putting your work on hold like that, but it wouldn't be good if we make changes to the implementation in this area, and it turns out to be slightly different from what we actually want in the end.

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[Yusufihsangorgel]

Not at all, and I've withdrawn the front-end offer over on the issue. The branch stays where it is.