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.

typeCheckPass(checker, null);
      typeCheckPass(checker, 'foo');
      typeCheckPass(checker, 123);
      checker = PropTypes.oneOfType([
        PropTypes.shape({a: PropTypes.number.isRequired}),
        PropTypes.shape({b: PropTypes.number.isRequired}),
      ]);
      typeCheckPass(checker, {a: 1});
      typeCheckPass(checker, {b: 1});
    });
    it('should be implicitly optional and not warn without values', function() {
      typeCheckPass(
        PropTypes.oneOfType([PropTypes.string, PropTypes.number]), null
      );
      typeCheckPass(
        PropTypes.oneOfType([PropTypes.string, PropTypes.number]), undefined
      );
    });
    it('should warn for missing required values', function() {
      typeCheckFail(
        PropTypes.oneOfType([PropTypes.string, PropTypes.number]).isRequired,
        null,
        requiredMessage
      );
      typeCheckFail(
        PropTypes.oneOfType([PropTypes.string, PropTypes.number]).isRequired,
        undefined,
        requiredMessage
      );
    });