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 missing required values', function() {
      typeCheckFail(
        PropTypes.shape({key: PropTypes.number}).isRequired,
        null,
        requiredMessage
      );
      typeCheckFail(
        PropTypes.shape({key: PropTypes.number}).isRequired,
        undefined,
        requiredMessage
      );
    });
  });
  describe('Symbol Type', function() {
    it('should warn for non-symbol', function() {
      typeCheckFail(
        PropTypes.symbol,
        'hello',
        'Invalid prop `testProp` of type `string` supplied to ' +
        '`testComponent`, expected `symbol`.'
      );
      typeCheckFail(
        PropTypes.symbol,
        function() { },
        'Invalid prop `testProp` of type `function` supplied to ' +
        '`testComponent`, expected `symbol`.'
      );
      typeCheckFail(