Add late class validation warnings for statics

ES6 classes won't have an early validation step. Therefore I added some
extra validation later on in the process. These would've normally have
been caught by createClass in classic React.
Do we have tests that validate the inverse, that using valid `propTypes` that fail the check are handled? I think this is the first I've seen of us using proptypes with classes (though I might have just missed that in the duplication of tests)