Private Static Accessors (Stage 3)

## Info

Follow up to https://github.com/babel/proposals/issues/55.

**Proposal Status**: Stage 3 @ [May 2018 TC39 meeting](https://github.com/tc39/agendas/blob/master/2018/05.md)
**Meeting Slides**: [Status Class Features](https://docs.google.com/presentation/d/1YzFr7EIGiX2YagfFMjkI-lVR6ouoRfPbTNLY--NGbC4/edit?usp=sharing)
**Proposal Repo**: [tc39/proposal-static-class-features](https://github.com/tc39/proposal-static-class-features/)

_Article on class features subject by @syg: ["The Semantics of All JS Class Elements"](https://rfrn.org/~shu/2018/05/02/the-semantics-of-all-js-class-elements.html)_

**Prior Work**:

+ Static class features (https://github.com/babel/babel/pull/8205)
+ Private methods (https://github.com/babel/babel/pull/8654)
+ Private accessors (https://github.com/babel/babel/pull/9101)
+ Private static methods (https://github.com/babel/babel/pull/9446)

cc @robpalme @littledan @nicolo-ribaudo

Start by reading the linked proposals issue and the tc39/proposal-static-class-features repository, then review the prior Babel pull requests listed in the issue. The issue does not identify a target file, test, or acceptance criteria, so the completed state is not defined from the available information.

Written by the indexing model from the issue text.