Allow "field promotion" to apply to abstract getters.

A quirk of analyzer and CFE implementations is that they resolve
property gets such as `foo.bar` to specific field or getter
declarations that may be not be directly defined on the target type;
for example if `foo` has type `B`, and `B` extends `A`, and `A`
contains a field called `bar`, then `foo.bar` is considered to refer
to `A.bar`, for example:

    class A {
      int? bar;
    }
    class B extends A {}
    f(B foo) {
      print(foo.bar); // Resolved to `A.bar`.
    }

This is in contrast with the language specification, which makes a
clean distinction between class _declarations_ and the _interfaces_
implied by those declarations. While a class declaration can contain
(among other things) both getters and fields, which might be concrete
or abstract, an interface doesn't distinguish between getters and
fields, and is inherently abstract.

The advantage of the analyzer/CFE approach is that it allows more
intuitive error messages and "go to definition" behavior, which
benefits users. But it has some ill-defined corner cases involving
complex class hierarchies, because not every property access can be
resolved to a unique declaration (sometimes a getter is multiply
inherited from multiple interfaces, for example). The language spec
approach has the advantage of being well-defined and consistent even
in situations involving complex class hierarchies.

When I initially implemented field promotion, I took advantage of this
quirk of the analyzer and CFE implementations, so that I could make
property gets that refer to field declarations promotable, while
keeping property gets that refer to abstract getter declarations
non-promotable. This caused unpredictable behaviors in the ill-defined
corner cases. It also meant that in certain rare cases, a property
access might not be promoted even when it would be sound to do so
(e.g. a property access might refer to a private abstract getter
declaration, but the only concrete _implementation_ of that abstract
getter was a final field).

This CL changes the rule for promotability so that any get of a
private property is considered promotable, provided that the
containing library doesn't contain any concrete getters, non-final
fields, or external fields with the same name. It no longer matters
whether the private property refers to a field or a getter. This rule
is simpler than the old rule, restores the spec's clean distinction
between class declarations and interfaces, and allows more promotions
without sacrificing soundness.

For additional details please see the breaking change announcement at
https://github.com/dart-lang/sdk/issues/54056, as well as the original
change proposal at https://github.com/dart-lang/language/issues/3328.

Fixes https://github.com/dart-lang/sdk/issues/54056.
Fixes https://github.com/dart-lang/language/issues/3328.

Change-Id: I38ffcb9ecce8bccb93b1b2586a1222a0fb1005a7
Bug: https://github.com/dart-lang/sdk/issues/54056
Bug: https://github.com/dart-lang/language/issues/3328
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/337609
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: Lasse Nielsen <lrn@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>
Reviewed-by: Johnni Winther <johnniwinther@google.com>