[ffigen] Dart API: Specialize Declarations further #2592
Description
DeclarationsFilter and Declaration are generic APIs that work on all kinds of language features currently. However, this can be somewhat confusing because not everything has members. We should consider properly splitting things.
(N.b. #2578 remes DeclarationsFilter to Declarations and introduces subtypes, but still exposes renameMember and shouldIncludeMember.)