fix($animate): invalid CSS class names should not break subsequent elements

[petebacondarwin]

The postDigest handler was not being added if the first element in
to modify the CSS classes contained invalid CSS class names. This meant
that subsequent valid CSS class changes were not being handled since we
were not then adding the handler for those correct cases.

Closes #12674

[petebacondarwin]

Here is the Plunker showing the issue with the fixed version of Angular: http://plnkr.co/edit/Zux7ICpKvsh6oa25vlzr?p=preview

[petebacondarwin]

The diff looks more complicated than it is.

I pulled two functions, updateData and handleClassChanges, from out of the addRemoveClassesPostDigest function.

This means that we are not having to recreate them on every call and also makes the addRemoveClassesPostDigest easier to grok.

[Narretz]

Without the refactor, it would be easier to see what actually changed semantically, though ;)

[matsko]

LGTM. Excellent work.

[petebacondarwin]

@Narrate you're right. I should have made two commits

[petebacondarwin]

Here is the plunker with the most recent fix: http://plnkr.co/edit/ppS7j623Wz4DifRRBjtu?p=preview

[petebacondarwin]

@Narretz - there I have broken the PR into two commits.

[Narretz]

@petebacondarwin I almost don't dare to say it, but ... it's actually refactor, not refact.

[petebacondarwin]

Doh!

[petebacondarwin]

@Narretz shouldn't you be asleep?