Run Babel asynchronously in fixtures

[nicolo-ribaudo]

Q	A
Fixed Issues?	Fixes #1, Fixes #2
Patch: Bug Fix?
Major: Breaking Change?
Minor: New Feature?
Tests Added + Pass?	Yes
Documentation PR Link
Any Dependency Changes?
License	MIT

This was extracted from #13414: by running our tests asynchronously, they can also run ESM plugins. The second commit verifies that it works by converting an existing test-only plugin from CJS to ESM.

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/52239/

[nicolo-ribaudo]

The failure is related, it looks like our logic to detect if dynamic import is supported was broken 😬

[liuxingbaoyu]

I remember having a similar problem when I was investigating jest-runner, you can try eval("import()").
Because it is a keyword.
😄

[nicolo-ribaudo]

Yes, but import is in a different file that we require in a try/catch block, so if it was a parsing error would be suppressed and correctly handled 🤔

[liuxingbaoyu]

This is related to jest-runner, but not only a jest-runner problem, our supportsESM does not actually call to test whether import() is available, so it will throw an exception when used.

Oh no, node crashes when we actually call it.

[nicolo-ribaudo]

I'm so annoyed that we have to hard-code the Node.js version, but there is no other synchronous way to know if dynamic import is supported in the current Node.js version. I used this code to test:

// demo.js
console.log("syntax is ok");
import("./.eslintrc.cjs").then(console.log, console.error);

[liuxingbaoyu]

Maybe we can use child processes!
😉

[nicolo-ribaudo]

Probably it's not worth it for this simple check 😬