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.

);
      typeCheckFail(
        PropTypes.string,
        0,
        'Invalid prop `testProp` of type `number` supplied to ' +
        '`testComponent`, expected `string`.'
      );
      typeCheckFail(
        PropTypes.string,
        {},
        'Invalid prop `testProp` of type `object` supplied to ' +
        '`testComponent`, expected `string`.'
      );
      typeCheckFail(
        PropTypes.string,
        Symbol(),
        'Invalid prop `testProp` of type `symbol` supplied to ' +
        '`testComponent`, expected `string`.'
      );
    });
    it('should fail date and regexp correctly', function() {
      typeCheckFail(
        PropTypes.string,
        new Date(),
        'Invalid prop `testProp` of type `date` supplied to ' +
        '`testComponent`, expected `string`.'
      );
      typeCheckFail(
        PropTypes.string,
        /please/,