Implement class fields Stage 3 proposal

[fatcerberus]

The class fields proposal is Stage 3 and seems to be implemented in at least V8 (Node 12, Chrome 74):
https://github.com/tc39/proposal-class-fields

It'd be awesome to have these in CC, I currently enjoy fields thanks to TypeScript, but would be awesome to be able to use them in vanilla JS too. I thought there was already an issue tracking this, but I couldn't find anything after several searches, so I'm posting a ticket now.

[zenparsing]

Thanks @fatcerberus, I agree!

[hax]

I believe class fields proposal have too many issues [1] and should be withdraw. Having two implementations (V8, Babel 7) is enough for testing or trying, I don't expect any new implementation which just make more sunk cost for the whole community.

• [1] See my slide https://johnhax.net/2019/class-fields/slide?netease#1 to check the issues of the proposal, sorry the slides is too long (275 pages and need 3 hours to have a full speech) but otoh still too simple (not easy to catch without futher explaining).

[fatcerberus]

It's already shipped in both Chrome and Node.js. Regardless of how you feel about the proposal (I don't personally have any issues with it, other than the weird # sigil but I ultimately prefer that over _ which I actively avoid so meh), it's way too late to stop that train now.

[rhuanjl]

Thinking about how this could be implemented:

1. public fields I think could be implemented with a hack in the parser:
   • catch them in the class body and store them in an array
   • when parsing the constructor iterate the array and add in an op for each one
2. Private fields though are obviously more complex:
   • trying to treat them as "normal" properties would likely create a horrible mess with special cases for them everywhere
   • I figure instead some kind of getPrivate/setPrivate op codes could be the way to go

I'm not sure if I'll have time to do this any time soon but I might, would this issue accept a PR if I can make it?