When I run this using php-src, I see:

```Warning: Class __PHP_Incomplete_Class has no unserializer in /data/users/seanc/fbcode2/hphp/test/slow/serialize/with_nonexistant_serializable_class.php on line 9
object(__PHP_Incomplete_Class)#2 (1) {
  ["__PHP_Incomplete_Class_Name"]=>
  string(3) "Bar"
}```

At a minimum, we should have a warning in this case.