Key warnings should show component stack

[sophiebits]

In #6771 I made it so that proptypes warnings show the stack of components where the warning occurred. We should do the same for key warnings (when no key is provided):

Each child in an array or iterator should have a unique "key" prop.

It shouldn't be too hard to use ReactComponentTreeDevtool.getCurrentStackAddendum in the same way that proptypes warnings are now using it and to update the unit tests so that they pass with the new format.