DocumentEvents should stop extending ElementEvents

[jacob314]

Now that Document is not trying to be an Element, DocumentEvents should stop extending ElementEvents

[blois]

The Document-Element thing is being tracked in issue http://code.google.com/p/dart/issues/detail?id=6806

I'm not sure what the proper solution is, but put a note in #­6806 that the events need to match the element hierarchy.

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Added Area-HTML label.

[efortuna]

Added this to the M3 milestone.

[blois]

Added Fixed label.