test: add test for experimental ESM modules

[bcoe]

Adds test demonstrating using c8 for collecting coverage for experimental modules (see conversation here).

I've been meaning to add this test for a while, but haven't had many OSS cycles... it's cool to see it work after months of noodling on V8/Node.js.

Background

The new module system in Node.js does not support require.extensions; while there is some work in motion to support similar extension mechanisms, it's unlikely to exist in time for --experimental-modules to be un-flagged.

c8 collects coverage using the inspector and the NODE_V8_COVERAGE environment variable, this is built into Node.js itself and does not require extending loaders.

Checklist

• npm test, tests passing
• npm run test:snap (to update the snapshot)
• tests and/or benchmarks are included

CC: @MylesBorins, @joyeecheung, @guybedford, @GeoffreyBooth, @coreyfarrell (CC: party 🎉 woo).

[coreyfarrell]

I tested this with https://github.com/coreyfarrell/c8-test, worked under node.js 11.13.0 but not 10.15.3. Is c8 with experimental-modules under node.js 10 not possible?

The test repo does equivalent testing with nyc using the esm module for easy comparison, the results under node.js 11 seem reasonable for both.

[bcoe]

@coreyfarrell it should work under 10, but you'd need to play around with wrapper-length I believe, might need to fiddle with a couple other things.

[coreyfarrell]

@bcoe we recently saw a change in node.js 11.11.0 which switched require from vm.runInThisContext to vm.compileFunction, is this the reason the needing to fiddle with wrapper-length?

[coreyfarrell]

Odd, if I set "wrapper-length": 0 in the config for c8 it works for node.js 10 and 11.

[bcoe]

@coreyfarrell the Node@10 issues should be addressed by the following:

nodejs/node@23ea7ee