Record: Include Dartdoc on Record fields if they are associated to a typedef

[rrousselGit]

Consider:

typedef Foo = ({
  /// Hello world
  int a,
});

It'd be useful if foo.a showed the record's Dartdoc.

[bwilkerson]

I like the idea.

Unfortunately, I don't think there's any way to do this in the analyzer's current semantic model. As far as I can tell there isn't any element representing the field of the record to which we could attach the documentation.

@johnniwinther @stereotype441 @scheglov Another situation in which our support for structural types would need to be enhanced.

[FMorschel]

Noting here too that #60332 is open and should probably be tackled with this.