Implement class features in estree

[JLHwung]

Q	A
Fixed Issues?	Fixes #12282, fixes #12105
Patch: Bug Fix?
Major: Breaking Change?
Minor: New Feature?	Yes
Tests Added + Pass?	Yes
Documentation PR Link
Any Dependency Changes?
License	MIT

This PR implements https://github.com/estree/estree/blob/master/experimental/class-features.md given that class features are stage 3 and shipped in several engines (V8, JSC, Gecko).

Note that ESLint does not support class features yet because Acorn has not yet implemented this proposal, though it supports a variant AST shape via acorn-class-features.

Eventually Acorn will support the AST shape supported in this PR since once class features are merged to ESTree. Hence we went ahead and implement this proposal in the estree plugin.

Support of ESTree class features is still experimental, Babel may change the class features AST if, when the ESTree proposal merged, the AST shapes differs, though it is very unlikely as far as I know.

This PR includes commits in #12375.

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[kaicataldo]

Support of ESTree class features is still experimental, Babel may change the class features AST if, when the ESTree proposal merged, the AST shapes differs, though it is very unlikely as far as I know.

Do we know how we want to handle a situation like this? Are we going to wait until major versions to change the AST or add options or something? Just want to make sure we don't break any plugins that rely on the current AST shape.

[nicolo-ribaudo]

So far we have always considered these changes as "new features" and shipped them in minors enabled by default: the contract of the "estree" option is "I want Babel to generate an AST that respects the ESTree spec", so if the ESTree spec changes not aligning with it is a bug (similarly, when the ECMAScript spec changes we introduce the breaking change to align as a bugfix).

However, I wouldn't be opposed to something like ["estree", { "classFeatures": true }] until Babel 8.

[kaicataldo]

Code looks good to me - just a few questions for clarity!

[JLHwung]

The class features ESTree proposal has been moved to stage 3 recently: estree/estree#242, which means unless the spec undergoes significant change, the current ESTree AST shape will not change. See also https://github.com/estree/estree/blob/master/stage3/README.md

[nicolo-ribaudo]

Q: Does eslint support this natively now?

[kaicataldo]

I haven't been involved in a few months, but, last I checked, ESLint's policy is to not crash on stage 3 syntax. Rules aren't updated for new syntax until they hit stage 4.

[ljharb]

No, sadly they're still insisting on waiting til stage 4 to support things that have already shipped in browsers and node.

[kaicataldo]

I have a few small suggestions for improving the clarity of comments, but the code LGTM!

[JLHwung]

Since the estree private brand checks proposal is also moved to stage 3. I just add a test case for private-in.