[WIP] Add support for class member variable

I've submitted this PR as WIP to get feedback, **the code does not work for now**

I am looking for:
- feedback on the implemenation,
- how to debug (currently I'm doing make + node debug which is not the most user friendly workflow),
- what kind of test should be added.

/cc @arv 

TODO:
- [ ] address feedback
- [x] writer: annotations
- [ ] assert type for all binary =

Can I ask you the rationale ?

When I write Dart code I usually use the "short" version but it seems like a lot of devs extract the length from the loop in JS.