This should possibly be in an override of `__construct()`, which call the parent... Just thinking to kill it earlier.

Something like:
```php
class TextDatastreamValidator extends DatastreamValidator {
  public function __construct(IslandoraFedoraObject $object, $datastream, array $params = array()) {
    // Dunno if doing a "count" on params might make more sense?
    if (!isset($params)) {
      throw new InvalidArgumentException('$params must contain two values to instantiate a TextDatastreamValidator.');
    }
    parent::__construct($object, $datastream, $params);
  }
  [...]
}
```