Add Support for Public Class Fields

[davydotcom]

It is becoming common place to use class fields and this is not yet supported from ES6 to ES5 conversion via closure compiler

https://github.com/tc39/proposal-class-fields

[rahbari]

Also Static Class Fields:
https://github.com/tc39/proposal-static-class-features/

[teppeis]

They are stage 3 features now.
https://github.com/tc39/proposals

[rahbari]

I wish to see this feature soon in closure compiler, having static fields helps a lot in class feature encapsulation.

class A {
    static a = 1;
}

instead of:

class A {}
A.a = 1;

[brad4d]

We've been looking forward to this feature making it to stage 4, also.
We're not ready to implement it in closure-compiler yet, but it is on our minds.

[justinfagnani]

@brad4d does a feature have to make it to stage 4 before implementing? Browsers will generally implement before stage 4, and all the other compilers I know of support class fields.

[ChadKillingsworth]

We've implemented features at stage 3 before. It's just a matter of priority or someone willing to contribute the work.

[arv]

At stage 3 now and shipping in V8.

https://github.com/tc39/proposal-class-fields
https://twitter.com/_gsathya/status/1100845457876541442

[blickly]

Exciting!

Created internal issue: http://b/127859638

[brad4d]

@justinfagnani
I'm actively working on an official SLO for handling new features.
It still requires review, but the current draft calls for us to start working on stage 3 features when we see that browsers have started adding support. (Lots of definition needed of what that means exactly.)

@arv
One thing that I find discourages me from prioritizing class fields over other stage 3 features, like BigInt, is that there still are no official tests associated with the proposal according to the TC39 active proposals page.
http://go/gh/tc39/proposals/blob/master/README.md

We would like to have these to base our unit tests on.
Do you know why there aren't any yet?
Thanks.

[justinfagnani]

@brad4d there are a ton of tests for public, private, and static fields and methods: https://github.com/tc39/test262/tree/master/src/class-elements

[brad4d]

@justinfagnani
Thanks for pointing out those tests.
I guess the page I pointed to needs to be updated.

[loadedsith]

If/when this ships, will it require an opt-in? Perhaps via @language ECMASCRIPT_2017?