Use import.meta.url in EXPORT_ES6 mode

[curiousdannii]

Follow on to #5368. When in EXPORT_ES6 mode import.meta.url should be used. In browsers document.currentScript will probably still work, but __dirname will not work in Node. import.meta.url is supported in both, and it's cleaner and safer.

[saschanaz]

https://aremodulesready.com/ The availability for import.meta.url seems fairly limited yet 😢

[curiousdannii]

It was just added in Firefox 62. Edge support is still lacking. Detecting its support could be tricky? I'm not sure if it will throw a syntax error in browsers which don't support it.

At the very least it's needed for Node, and supported in Node. Maybe leave browsers till later when it's reached stage 4.

[saschanaz]

It's tricky because import.meta has a special syntactic form with a keyword so unsupported browsers will throw an early SyntaxError. Waiting Stage 4 makes sense, though.

[stale]

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