Ensure const modules in async bundles are wrapped

[marcins]

↪️ Pull Request

This fixes a case where an async module only exports a constant, and it doesn't end up getting registered.

We had a use case where there were lots of async bundles containing just string exports for a PDF renderer, and a big switch case that would dynamically import fonts on demand. These small async bundles weren't registering their exports.

Pseudo example based on the real pattern - where the real values could vary in size between several hundred bytes and 60KB+, hence deferred loading:

// fonts.ts
export const fonts = {
   'abc': () => import('./abc'),
   'def': () => import('./def')
}

// abc.ts
export const value = "abc";


// consumer.ts
import { fonts } from './fonts';

const { value } = await fonts['abc']();

🚨 Test instructions

• Added new integration test that was failing before the fix.
• Verified internally that the affected async bundles correctly register their assets

✔️ PR Todo

• Added/updated unit tests for this change
• Filled out test instructions (In case there aren't any unit tests)
• Included links to related issues/PRs

[mattcompiles]

Does the asset still get inlined in this case? It wouldn't be ideal if you ended up with something like:

import('./async').then(m => console.log('async value'));

[marcins]

Good question, it doesn't seem to - I assume the static analysis can't track the usage to that level anyway?

From the bundle in the integration test:

(parcelRequire("8Vicm")).then((m)=>console.log(m.value));

[marcins]

I've added additional verification in the integration test.