Fix optional argument for topological sort, target ES6

PR: Fix optional argument for topological sort, target ES6. Thanks to @CrispyBacon1999 for this contribution!

I'm making this a major version bump because the target language for the JS build has changed from ES5 (var) to ES6 (const, let). While for the most part this will not cause breakages, there's a slight possibility that it would, in cases where downstream products run in an environment that does not support ES6.