Add ES6 exports

[curiousdannii]

Fixes #6284

The code change is very simple. But I'm not sure about tests.

What version of Node is bundled in emsdk?
Can I just make a simple test that checks importing the built module works, or does it need more rigorous testing?

Node requires ES6 modules to have the extension .mjs. Should this mode automatically export files as a.out.mjs instead of .js? And it should support -o out.mjs. Maybe that could even automatically turn on EXPORT_ES6 if we wanted to be clever!

[kripken]

Most people just use the already-installed node, I think. Looking in our docs, we don't mention a minimum version currently. Would this require a recent version?

[curiousdannii]

From 8.5, but it's behind a flag which would make testing more difficult. And actually it wouldn't be needed in a emsdk, just the testing environment.

[kripken]

I see. Well, if it's behind a flag I guess we can wait on testing it. Once it's enabled in a stable node release we can make sure that's on the bots and add a test. How about writing up the test now, though (in other, I guess) and leaving it disabled, so in the future we just need to enable it?

[kripken]

Should we land this? We can figure out testing later I guess.

[curiousdannii]

Sorry, I was planning to write tests, but haven't had much time.

[xtuc]

I can also review it and actually test it as well.

[xtuc]

I'm testing with: ./emcc.py -s MODULARIZE=1 -s EXPORT_ES6=1 -s WASM=1 t.c.

I noticed that import are:

(import "env" "memory" )
(import "global" "NaN" (global f64))
;; ...

In order to work with JavaScript bundlers (Webpack and Rollup atm) and probably Node, the imports module should be a local file. The glue needs to use a named export then.

It seems to be a bigger change to the glue, but I think that would be worth it in the long run.

Node requires ES6 modules to have the extension .mjs

I think it's fine, it will also wok with .js for now.

[kripken]

Ok, thanks @curiousdannii and @xtuc , let's merge this in for now. @curiousdannii, if you get a chance to add tests later that would be great!