Memoize class binding when compiling private methods and static elements

[JLHwung]

Q	A
Fixed Issues?	Fixes #15696
Patch: Bug Fix?	Yes
Tests Added + Pass?	Yes
Documentation PR Link
Any Dependency Changes?
License	MIT

In this PR we memoise the class binding when it is referenced in private methods or static elements, either via this or the class id. Previously we replaced this with class id, but as #15696 point out,

Class declarations in JavaScript generate an outer binding name, which turns out to be mutable (and can be reassigned). Separately, JavaScript also generates an immutable inner binding name that is only in scope within the class body.

it will fail when the outer binding is reassigned.

I suggest reviewing this PR by commits as there are many fixture changes.

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/54907/

[JLHwung]

When ref is not defined and classRefFlags.ForInnerBinding is not enabled, e.g. compiling a plain class class C {} without private methods / static elements, we will generate a uid identifier but we do not insert it to the final AST. In this case the memoizer variable _class0, _class1, etc. is not continuous.

[nicolo-ribaudo]

Is only generating the extra ref when the class is reassigned doable?

[nicolo-ribaudo]

I pushed some commits to do #15701 (comment), wdyt about this approach?

[JLHwung]

@nicolo-ribaudo Thanks for working on this. I didn't wrap it under the reassignment check because

• It generally does not work with the case where the reassignment is introduced by a plugin. The replacement methods like NodePath#insertAfter or NodePath#replaceWith will not requeue the binding node. And it's asking a bit too much for plugin authors to do so.
• The extra assignment does not necessarily bloat the output size. When the class id is long and class bindings are frequently used, the reassignment is doing a favor for the compressor as all the reference identifiers are now able to be mangled. Because of the compressor I didn't go ahead and fix the uuid continuity.

Btw TypeScript memoizes the class binding without checking if it is reassigned, too: https://www.typescriptlang.org/play?#code/MYGwhgzhAEBiD29oG8BQ0PQgFzNglsNAMQjQC8ciA3KgL5A

[nicolo-ribaudo]

👍 I reverted my changes, the code is good

[JLHwung]

✅ Just rebased this PR and it looks good to me.