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.

'Invalid prop `testProp` of type `String` supplied to ' +
        '`testComponent`, expected instance of `' + personName + '`.'
      );
      typeCheckFail(
        PropTypes.instanceOf(Date),
        {},
        'Invalid prop `testProp` of type `Object` supplied to ' +
        '`testComponent`, expected instance of `' + dateName + '`.'
      );
      typeCheckFail(
        PropTypes.instanceOf(RegExp),
        {},
        'Invalid prop `testProp` of type `Object` supplied to ' +
        '`testComponent`, expected instance of `' + regExpName + '`.'
      );
      typeCheckFail(
        PropTypes.instanceOf(Person),
        new Cat(),
        'Invalid prop `testProp` of type `Cat` supplied to ' +
        '`testComponent`, expected instance of `' + personName + '`.'
      );
      typeCheckFail(
        PropTypes.instanceOf(Person),
        Object.create(null),
        'Invalid prop `testProp` of type `<<anonymous>>` supplied to ' +
        '`testComponent`, expected instance of `' + personName + '`.'
      );
    });
    it('should not warn for valid values', function() {
      function Person() {}