The pluginBase class contains a t() method

Code should not call the global t() function anymore but use a t method on the different objects. For plugins
a new t() method was added on the base class.

Before

use \Drupal\Core\Plugin\PluginBase;

class Example extends PluginBase {

  public function someFunction() {
    return t('Some description'):
  }

}

After

use \Drupal\Core\Plugin\PluginBase;

class Example extends PluginBase {

  public function someFunction() {
    return $this->t('Some description'):
  }

}

See also https://drupal.org/node/2079611