Use native Node.js functions when available

[nicolo-ribaudo]

Q	A
Tests Added + Pass?	Yes
Documentation PR Link
Any Dependency Changes?
License	MIT

This is the same as #12439 but for fs.mkdirSync(path, { recursive: true }) and for process.allowedNodeEnvironmentFlags: we use the native implementation when available, and fallback to the polyfill when it's not.

This allows us to more easily prepare a Babel 8 prerelease, in the spirit of #12440, by simply toggling the BABEL_8_BREAKING env variable.

• fs.mkdirSync(filepath, { recursive: true }) is polyfilled at build time as parseFloat(process.versions.node) >= 10.12 ? fs.mkdirSync : require("make-dir").sync (we can't easily feature-check because we need to polyfill the recursive option)
• process.allowedNodeEnvironmentFlags is polyfilled as process.allowedNodeEnvironmentFlags || require("node-environment-flags").

Note that during local development those polyfills aren't injected since they aren't needed, but we are testing them on CI (with Node.js 6 and 8).

[nicolo-ribaudo]

This polyfill was already there, I only moved it.

[babel-bot]

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[nicolo-ribaudo]

I opened this PR because I already had it ready, but if you don't think that the added co.plexoty in babel.config.js file is worth it (I'm not 100% sure) I am ok with closing this.

[hzoo]

so this doesn't remove the dep, just makes a plugin that toggles on or off the require to the dep based on node version - sounds fine since we will remove in v8 - and CI test should cover it? assuming that this shouldn't break anything in a patch

[nicolo-ribaudo]

Exactly, we are testing both the behaviors: on Node.js > 10 we test the new built-in functions, while the old functions are tested on Node.js 6 and 8.

The dependency is still there (because of the polyfill), but this PR makes it so that we will only have to update package.json for Babel 8 and not the actual code.