Expose information about import assertions

[LarsDenBakker]

I am looking into polyfilling the new upcoming module types (JSON, CSS, HTML etc.). To do this it would be great if we can find out in a lightweight way whether an import is done using an import assertion. Is this something that could be added to ex module lexer?

For an import like this:

import foo from 'bar' assert { type: 'json' } 

And this would tell use that the import assertion is JSON.

The proposal for import assertions is here: https://github.com/tc39/proposal-import-assertions

[guybedford]

@LarsDenBakker I think we should build this directly into the core project, just like SystemJS supports - https://github.com/systemjs/systemjs/blob/main/src/extras/module-types.js.

Steps:

1. Support import assertions in ES module lexer (let me work on a PR now).
2. Integrate the module-types logic into the main repo here, assuming the constructed stylesheets polyfill like SystemJS does.

What is the status on HTML modules these days?

[LarsDenBakker]

Great to hear! Import assertions were stalling progress for a while, but JSON module is stage 3 and CSS modules is making progress https://groups.google.com/a/chromium.org/g/blink-dev/c/vCrJCQxNnzo/m/OAYI9cPWAQAJ?pli=1. I'm not sure about the current status of HTML modules

[LarsDenBakker]

Is it possible to include the start and end position of the import assertion? For my case I'd have to remove the actual import assertion from the code. es-module-shims will have to do this as well I think?

[guybedford]

@LarsDenBakker thanks for the suggestions - I've added these features in #74, review would be very welcome there.