Issue #2134861 by Gábor Hojtsy, amateescu: Field instance bundle is only accessible directly on the bundle property.

core/modules/config_translation/lib/Drupal/config_translation/ConfigFieldInstanceMapper.php

@@ -18,15 +18,20 @@ namespace Drupal\config_translation;
  */
 class ConfigFieldInstanceMapper extends ConfigEntityMapper {
 
+  /**
+   * Loaded entity instance to help produce the translation interface.
+   *
+   * @var \Drupal\field\FieldInstanceInterface
+   */
+  protected $entity;
+
   /**
    * {@inheritdoc}
    */
   public function getBaseRouteParameters() {
     $parameters = parent::getBaseRouteParameters();
     $base_entity_info = $this->entityManager->getDefinition($this->pluginDefinition['base_entity_type']);
-    // @todo Field instances have no method to return the bundle the instance is
-    //   attached to. See https://drupal.org/node/2134861
-    $parameters[$base_entity_info['bundle_entity_type']] = $this->entity->bundle;
+    $parameters[$base_entity_info['bundle_entity_type']] = $this->entity->targetBundle();
     return $parameters;
   }
 

core/modules/field/lib/Drupal/field/Entity/FieldInstance.php

@@ -597,6 +597,13 @@ class FieldInstance extends ConfigEntityBase implements FieldInstanceInterface {
     $this->bundle_rename_allowed = TRUE;
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  public function targetBundle() {
+    return $this->bundle;
+  }
+
   /*
    * Implements the magic __sleep() method.
    *

core/modules/field/lib/Drupal/field/FieldInstanceInterface.php

@@ -32,4 +32,12 @@ interface FieldInstanceInterface extends ConfigEntityInterface, FieldDefinitionI
    */
   public function allowBundleRename();
 
+  /**
+   * Returns the name of the bundle this field instance is attached to.
+   *
+   * @return string
+   *   The name of the bundle this field instance is attached to.
+   */
+  public function targetBundle();
+
 }