fix(StaticMetadataExtractor): Map members annotations to all annotations

@blois I would appreciate if you can review this.

The changes are in the second commit. The first one is only about bringing some consistency in code style vs the rest of a.dart.

One question for you: why do we need to do fancy stuff (check if the class has a map member, assignable to a Dart map, ...) instead of looking only at the base class ?

I'll rebase #861 on top of this when this (eventually) gets merged.