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.

typeCheckPass(PropTypes.element, null);
      typeCheckPass(PropTypes.element, undefined);
    });
    it('should warn for missing required values', function() {
      typeCheckFail(PropTypes.element.isRequired, null, requiredMessage);
      typeCheckFail(PropTypes.element.isRequired, undefined, requiredMessage);
    });
  });
  describe('Instance Types', function() {
    it('should warn for invalid instances', function() {
      function Person() {}
      function Cat() {}
      var personName = Person.name || '<<anonymous>>';
      var dateName = Date.name || '<<anonymous>>';
      var regExpName = RegExp.name || '<<anonymous>>';
      typeCheckFail(
        PropTypes.instanceOf(Person),
        false,
        'Invalid prop `testProp` of type `Boolean` supplied to ' +
        '`testComponent`, expected instance of `' + personName + '`.'
      );
      typeCheckFail(
        PropTypes.instanceOf(Person),
        {},
        'Invalid prop `testProp` of type `Object` supplied to ' +
        '`testComponent`, expected instance of `' + personName + '`.'
      );
      typeCheckFail(
        PropTypes.instanceOf(Person),
        '',