Right now we validate some of our proptype definitions, but we don't surface the issue until that component class is consumed. We should instead validate these, in `__DEV__` only as warnings when the class in created. This means that a call to `React.PropTypes.onOf('foo')` results in an immediate warning logged.

This is a relatively contained bit of code so would be a good candidate for somebody looking to start contributing to React.

it('should warn for required valid types', function() {
      typeCheckFail(
        PropTypes.shape({key: PropTypes.number.isRequired}),
        {},
        'Required prop `testProp.key` was not specified in `testComponent`.'
      );
    });
    it('should warn for the first required type', function() {
      typeCheckFail(
        PropTypes.shape({
          key: PropTypes.number.isRequired,
          secondKey: PropTypes.number.isRequired,
        }),
        {},
        'Required prop `testProp.key` was not specified in `testComponent`.'
      );
    });
    it('should warn for invalid key types', function() {
      typeCheckFail(PropTypes.shape({key: PropTypes.number}),
        {key: 'abc'},
        'Invalid prop `testProp.key` of type `string` supplied to `testComponent`, ' +
        'expected `number`.'
      );
    });
    it('should be implicitly optional and not warn without values', function() {
      typeCheckPass(
        PropTypes.shape(PropTypes.shape({key: PropTypes.number})), null
      );
      typeCheckPass(
        PropTypes.shape(PropTypes.shape({key: PropTypes.number})), undefined