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.arrayOf(PropTypes.instanceOf(Thing)),
        [new Thing(), new Thing()]
      );
    });
    it('should warn with invalid items in the array', function() {
      typeCheckFail(
        PropTypes.arrayOf(PropTypes.number),
        [1, 2, 'b'],
        'Invalid prop `testProp[2]` of type `string` supplied to ' +
        '`testComponent`, expected `number`.'
      );
    });
    it('should warn with invalid complex types', function() {
      function Thing() {}