Warn when using React.unmountComponentAtNode on a different React instance's tree

[sophiebits]

We should be able to look at see if the child has a data-react-id attribute but isn't rendered by this copy of React and consequently warn.

See https://groups.google.com/forum/#!topic/reactjs/mdwfKKJ0L2Q.

[jimfb]

Probably same with React.render, right? We would want to warn if you are rendering into another React's instance tree?

[sophiebits]

Sure. @crm416 had some related warnings in #2065.

[gaearon]

Fixed in #7456.