Added findDOMNode, as we move toward deprecating g

Added findDOMNode, as we move toward deprecating getDOMNode
Obviously need to clean this up and not mention elements here. And like we talked about, not completely sure if this is the best way to do isValidComponent checks (when we removed `isValidComponent` from the top level API there were some discussions about how it's a bit tricky especially moving forward as other ideas about what a component is/might be come into play)