Script error in shedule.js in IE9/Win7

[zzcypc]

Ping @KarolBuchta - please comment if you have additional info

I can't reproduce this without using a faulty polyfill. I recommend you try including bluebird first and any polyfills second.

Originally posted by @benjamingr in #1236 (comment)

[zzcypc]

In the case where the page first included the MutationObserver polyfill, in IE9, an exception will be thrown because the classList does not exist. I think that we should judge the existence of the classList at the same time as we judge the existence of the MutationObserver, which can cover more usage scenarios and the code can be more robust.

https://github.com/petkaantonov/bluebird/blob/master/src/schedule.js#L31