function getVisitorKeys(visitorKeys, node) {
	let keys = visitorKeys[node.type];

	if (!keys) {
		keys = vk.getKeys(node);
		debug(
			'Unknown node type "%s": Estimated visitor keys %j',
			node.type,
			keys,
		);
	}

	return keys;
}
Get the visitor keys of a given node.
@param {Object} visitorKeys The map of visitor keys.
@param {ASTNode} node The node to get their visitor keys.
@returns {string[]} The visitor keys of the node.