utilize classList when supported (refs #5087)

[mgol]

No description provided.

[rwaldron]

Thanks for the contribution! As owner of http://bugs.jquery.com/ticket/5087 I can assure you I haven't forgotten! The classList API is still a poor replacement, performance wise, when dealing with multiple classes per call.

[mgol]

Look at my last comment under http://bugs.jquery.com/ticket/5087, I created a few test cases which show we get a large performance boost after applying this patch... What am I missing?

[rwaldron]

That comment wasn't there when I visited the ticket just before closing the PR... I've owned the ticket and babysit the implementation schedule for years and I stand by these decisions:

1. There is no benefit to adding more code that does the same thing that jQuery already does
2. Any performance benefits are outweighed by 1
3. Multiple classes have been a performance issue
4. jQuery will not use classList until it accepts multiple class args to the add and remove APIs, at which time we'll remove the old way and completely replace it.

[mgol]

Well, multiple classes don't seem to be a performance issue anymore, at least it didn't seem so in my test cases on jsFiddle, even with the current limited classList API.

But I understand that as the className path is required even for jQuery 2.0 because of IE9, your first two points have settled it by themselves so I respect your decision.