Show where syntax error from with ESM

[outsideris]

Description of the Change

This is a kind of hack because node.js currently not support syntax errors in detail. See nodejs/node#49441
And This is another approach to #4557 .

I found node --unhandled-rejections=throw -e "import('syntax-error-file.mjs')" show syntax erroed file, linenumber, and code.

So, I try to run it with a file to import only when SyntaxError raised.

So, it will show like this:

$ mocha -ui qunit test*.mjs

file:///Users/outsider/Dropbox/projects/github/mocha/test2.mjs:5
test('test2a', => { // syntax error!
               ^^

SyntaxError: Unexpected token '=>'
    at Loader.moduleStrategy (internal/modules/esm/translators.js:145:18)
    at async link (internal/modules/esm/module_job.js:47:21)

And --unhandled-rejections=throw is default since Node.js 15.0.0

Alternate Designs

See #4557

Why should this be in core?

Before node.js itself support it, Users can find easily where a syntax error from.

Possible Drawbacks

It can be broken when Node.js itself supports it.

Applicable issues

close #4551

If we agreed with this approach, I will add tests for It.
I want to hear your opinion about this workaround.

[coveralls]

Coverage decreased (-0.3%) to 93.867% when pulling fa51418 on outsideris:esm-syntax-error into c667d10 on mochajs:master.

[juergba]

@outsideris IMO this solution goes too far. It's certainly a creative work-around, but we should not try to fix Node bugs in such a complex way as spawning a child-process.

I prefer the simpler approach of @giltayar which just injects the file name into the original error stack.

Btw I noticed that in Node v15.3.0 the stability of Node's ESM implementation has changed to Stable. Nice.

[outsideris]

I also know that. I don't have a strong opinion to use this approach.
I just made a proposal after digging to find a way where the syntax error is from.

Without this, we can inject the filename only which is not enough information for a large project.
Spawning a child process is a dirty way.

Btw I noticed that in Node v15.3.0 the stability of Node's ESM implementation has changed to Stable. Nice

Anyway, it is so good.

[outsideris]

Closed by #4557