When there are static variables like `propTypes` and `defaultProps`, we shouldn't export them until we have actually set them onto the `Component`. This may change if the current ES7 proposal for static class properties gets accepts, and we update to support it.
By exporting `propTypes` as a named export *as well* as attaching them to the "class", they can be explicitly imported by tests and other components in a way that aligns with the future ES6 module dependency graph, rather than as an arbitrary property that isn't statically verifiable. Doing both is great imo.
Thoughts?
