custom-elements seems to assume that html-imports adds .import to links before it actually does

[webcomponents]

custom-elements seems to assume that html-imports adds .import to links before it actually does

[TimvdLippe]

I think this is fixed in the latest version of the polyfill, where we wait for the load event if necessary:

custom-elements/src/CustomElementInternals.js

Lines 181 to 218 in 6374f37

	const gatherElements = element => {
	if (element.localName === 'link' && element.getAttribute('rel') === 'import') {
	// The HTML Imports polyfill sets a descendant element of the link to
	// the `import` property, specifically this is *not* a Document.
	const importNode = /** @type {?Node} */ (element.import);
	if (importNode instanceof Node) {
	importNode.__CE_isImportDocument = true;
	// Connected links are associated with the registry.
	importNode.__CE_hasRegistry = true;
	}
	if (importNode && importNode.readyState === 'complete') {
	importNode.__CE_documentLoadHandled = true;
	} else {
	// If this link's import root is not available, its contents can't be
	// walked. Wait for 'load' and walk it when it's ready.
	element.addEventListener('load', () => {
	const importNode = /** @type {!Node} */ (element.import);
	if (importNode.__CE_documentLoadHandled) return;
	importNode.__CE_documentLoadHandled = true;
	// Clone the `visitedImports` set that was populated sync during
	// the `patchAndUpgradeTree` call that caused this 'load' handler to
	// be added. Then, remove *this* link's import node so that we can
	// walk that import again, even if it was partially walked later
	// during the same `patchAndUpgradeTree` call.
	const clonedVisitedImports = new Set(visitedImports);
	clonedVisitedImports.delete(importNode);
	this.patchAndUpgradeTree(importNode, {visitedImports: clonedVisitedImports, upgrade});
	});
	}
	} else {
	elements.push(element);
	}
	};

Therefore, I am closing this issue.