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.

'string',
        'Invalid prop `testProp` of type `string` supplied to ' +
        '`testComponent`, expected an array.'
      );
    });
    it('should not warn when passing an empty array', function() {
      typeCheckPass(PropTypes.arrayOf(PropTypes.number), []);
    });
    it('should be implicitly optional and not warn without values', function() {
      typeCheckPass(PropTypes.arrayOf(PropTypes.number), null);
      typeCheckPass(PropTypes.arrayOf(PropTypes.number), undefined);
    });
    it('should warn for missing required values', function() {
      typeCheckFail(
        PropTypes.arrayOf(PropTypes.number).isRequired,
        null,
        requiredMessage
      );
      typeCheckFail(
        PropTypes.arrayOf(PropTypes.number).isRequired,
        undefined,
        requiredMessage
      );
    });
  });
  describe('Component Type', function() {
    beforeEach(function() {
      Component = React.createClass({
        propTypes: {
          label: PropTypes.element.isRequired,