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.

'`testComponent`, expected an object.'
      );
      typeCheckFail(
        PropTypes.objectOf(PropTypes.number),
        'string',
        'Invalid prop `testProp` of type `string` supplied to ' +
        '`testComponent`, expected an object.'
      );
      typeCheckFail(
        PropTypes.objectOf(PropTypes.symbol),
        Symbol(),
        'Invalid prop `testProp` of type `symbol` supplied to ' +
        '`testComponent`, expected an object.'
      );
    });
    it('should not warn when passing an empty object', function() {
      typeCheckPass(PropTypes.objectOf(PropTypes.number), {});
    });
    it('should be implicitly optional and not warn without values', function() {
      typeCheckPass(PropTypes.objectOf(PropTypes.number), null);
      typeCheckPass(PropTypes.objectOf(PropTypes.number), undefined);
    });
    it('should warn for missing required values', function() {
      typeCheckFail(
        PropTypes.objectOf(PropTypes.number).isRequired,
        null,
        requiredMessage
      );
      typeCheckFail(
        PropTypes.objectOf(PropTypes.number).isRequired,