Class Fields (Stage 3)

Original issue submitted by @babel-bot in https://github.com/babel/babel/issues/4408

Champions: @jeffmo (public class fields) @littledan (private + combined)
Spec Repo: https://github.com/tc39/proposal-class-fields
Spec Text: https://tc39.github.io/proposal-class-fields/
Slides: https://drive.google.com/file/d/0B-TAClBGyqSxWHpyYmg2UnRHc28/view

> Moved to Stage 3 at the July 2017 meeting: https://github.com/tc39/agendas/blob/master/2017/07.md (https://twitter.com/lbljeffmo/status/890679542007738368)

## Examples

```js
class C {
static x = 1, #y, [a];
z, #w = 2, [b];

a() {
this.z;
this.#w++;
#w; // #w is this.#w
}
}
```

## Parsing/ESTree
- [x] AST handled already in current `classProperties` and `classPrivateProperties`.
- [ ] May need to rename though (new plugin name).
- [x] make sure no "private computed" parse
- [ ] need to support comma separated properties.

## Transform
- [ ] combine the class-properties plugin + tbd private properties one

## Contacts
- [#proposal-class-fields](https://babeljs.slack.com/messages/proposal-class-fields) on Slack!
- @littledan
- @diervo (dval on Slack)
- @Qantas94Heavy

Start with the current classProperties and classPrivateProperties AST handling, then review the linked TC39 proposal and spec. The remaining checklist covers possible plugin renaming, comma-separated properties, and combining the class-properties and private-properties transforms; done means those Stage 3 requirements are addressed.

Written by the indexing model from the issue text.