Fix scopehositing with nested dynamic imports

[mischnic]

↪️ Pull Request

Closes #2300, @garthenweb

Probably introduced by #1682

When the assets are iterated on JSConcatPackager.start

parcel/packages/core/parcel-bundler/src/packagers/JSConcatPackager.js

Lines 32 to 49 in c670dec

	for (let asset of this.bundle.assets) {
	// If this module is referenced by another JS bundle, it needs to be exposed externally.
	let isExposed = !Array.from(asset.parentDeps).every(dep => {
	let depAsset = this.bundler.loadedAssets.get(dep.parent);
	return this.bundle.assets.has(depAsset) || depAsset.type !== 'js';
	});
	if (
	isExposed ||
	(this.bundle.entryAsset === asset &&
	this.bundle.parentBundle &&
	this.bundle.parentBundle.childBundles.size !== 1)
	) {
	this.exposedModules.add(asset);
	this.needsPrelude = true;
	}
	this.assets.set(asset.id, asset);

they are also added to the assets Map. For "normal" bundles bundle-loader is added here, but for dynamically imported bundles only actual static imports is added.
For dynamically imported bundles which need also need the bundle-loader, it gets added here (called by addAsset)

parcel/packages/core/parcel-bundler/src/packagers/JSConcatPackager.js

Lines 400 to 415 in c670dec

	async addBundleLoader(bundleType, parentAsset, dynamic) {
	let loader = this.options.bundleLoaders[bundleType];
	if (!loader) {
	return;
	}
	let bundleLoader = this.bundler.loadedAssets.get(
	require.resolve('../builtins/bundle-loader')
	);
	if (!bundleLoader && !dynamic) {
	bundleLoader = await this.bundler.getAsset('_bundle_loader');
	}
	if (bundleLoader) {
	// parentAsset.depAssets.set({name: '_bundle_loader'}, bundleLoader);
	await this.addAssetToBundle(bundleLoader);

, but addAssetToBundle didn't add it to the assets map, resulting in this error:

Cannot read property 'depAssets' of undefined 
    at JSConcatPackager.resolveModule (<work_space>/node_modules/parcel-bundler/src/packagers/JSConcatPackager.js:551:19)
    at CallExpression (<work_space>/node_modules/parcel-bundler/src/scope-hoisting/concat.js:152:28)
    at NodePath._call (<work_space>/node_modules/@babel/traverse/lib/path/context.js:53:20)
    at NodePath.call (<work_space>/node_modules/@babel/traverse/lib/path/context.js:40:17)
    at NodePath.visit (<work_space>/node_modules/@babel/traverse/lib/path/context.js:88:12)
    at TraversalContext.visitQueue (<work_space>/node_modules/@babel/traverse/lib/context.js:118:16)
    at TraversalContext.visitSingle (<work_space>/node_modules/@babel/traverse/lib/context.js:90:19)
    at TraversalContext.visit (<work_space>/node_modules/@babel/traverse/lib/context.js:146:19)
    at Function.traverse.node (<work_space>/node_modules/@babel/traverse/lib/index.js:94:17)
    at NodePath.visit (<work_space>/node_modules/@babel/traverse/lib/path/context.js:95:18)

💻 Examples

Fix scopehoisting for situations like this:

bundle   ----dynamic-import--->   bundle   ----!!dynamic-import!!--->   bundle

✔️ 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

[garthenweb]

Ha, I thought about these lines as well but was not able to put the pieces together ;)
I can prove that this solves it for my example as well, really cool that you found and fixed it! 🎉