perf($animate): don't join classes before it's necessary in resolveElementClasses
perf($animate): don't join classes before it's necessary in resolveElementClasses

In ngAnimate, we can't do this because the behaviour is exposed via the API. But
in core, we can avoid a bit of work.

ggershoni/angular.js,ggershoni/angular.js,ggershoni/angular.js,ggershoni/angular.js