getFlattenedTree can crash with <slot> elements without shadowDOM support

[WilcoFiers]

axe-core/lib/core/utils/flattened-tree.js

Lines 92 to 98 in f13ad36

	} else if (nodeName === 'slot') {
	realArray = Array.from(node.assignedNodes());
	if (!realArray.length) {
	// fallback content
	realArray = getSlotChildren(node);
	}
	var styl = window.getComputedStyle(node);

Line 93 here throws in Firefox, because assignedNodes isn't defined here. I think L92 should check that that function exists and otherwise just move on. @dylanb WDYT?

[dylanb]

@WilcoFiers Do you have an example of a page that fails on FF? I would like to validate that what FF is doing is correct before building in this workaround.

[WilcoFiers]

Ran into it while I was working on ESLint. The following throws in FF and Phantom:

fixture.innerHTML = '<button><slot></slot></button>';
axe._tree = axe.utils.getFlattenedTree(fixture);

[WilcoFiers]

This probably crashes on everything except Chrome. Updating the name.