Sets a particular value in the block settings.
@todo This doesn't belong here. Move this into a new base class in http://drupal.org/node/1764380. @todo This does not set a value in config(), so the name is confusing.
string $key: The key of PluginBase::$configuration to set.
mixed $value: The value to set for the provided key.
\Drupal\Component\Plugin\PluginBase::$configuration
public function setConfig($key, $value) {
$this->configuration[$key] = $value;
}