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.

var name = Thing.name || '<<anonymous>>';
      typeCheckFail(
        PropTypes.objectOf(PropTypes.instanceOf(Thing)),
        {a: new Thing(), b: 'xyz'},
        'Invalid prop `testProp.b` of type `String` supplied to ' +
        '`testComponent`, expected instance of `' + name + '`.'
      );
    });
    it('should warn when passed something other than an object', function() {
      typeCheckFail(
        PropTypes.objectOf(PropTypes.number),
        [1, 2],
        'Invalid prop `testProp` of type `array` supplied to ' +
        '`testComponent`, expected an object.'
      );
      typeCheckFail(
        PropTypes.objectOf(PropTypes.number),
        123,
        'Invalid prop `testProp` of type `number` supplied to ' +