Add missing t() function.

src/Plugin/search_api/processor/Property/EntityReferenceWithUriProperty.php

@@ -47,8 +47,8 @@ public function buildConfigurationForm(FieldInterface $field, array $form, FormS
     ];
     $form['require_all'] = [
       '#type' => 'checkbox',
-      '#title' => 'Require all terms',
-      '#description' => 'Only index related entities that have all the listed terms.',
+      '#title' => $this->t('Require all terms'),
+      '#description' => $this->t('Only index related entities that have all the listed terms.'),
       '#default_value' => $configuration['require_all'],
     ];
     // Store the target type of the field, it's a pain to get it when indexing.

src/Plugin/search_api/processor/Property/EntityReferenceWithUriProperty.php~

@@ -0,0 +1,62 @@
+<?php
+
+namespace Drupal\islandora\Plugin\search_api\processor\Property;
+
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\StringTranslation\StringTranslationTrait;
+use Drupal\search_api\Item\FieldInterface;
+use Drupal\search_api\Processor\ConfigurablePropertyBase;
+use Drupal\taxonomy\Entity\Term;
+
+/**
+ * Defines a "Entity Reference with URI" search property.
+ *
+ * @see \Drupal\islandora\Plugin\search_api\processor\EntityReferenceWithUri
+ */
+class EntityReferenceWithUriProperty extends ConfigurablePropertyBase {
+
+  use StringTranslationTrait;
+
+  /**
+   * {@inheritdoc}
+   */
+  public function defaultConfiguration() {
+    return [
+      'filter_terms' => [],
+      'require_all' => TRUE,
+      'target_type' => '',
+    ];
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildConfigurationForm(FieldInterface $field, array $form, FormStateInterface $form_state) {
+    $configuration = $field->getConfiguration();
+    $logger = \Drupal::logger('islandora');
+    $logger->info('<pre>' . print_r($configuration['filter_terms'], TRUE) . '</pre>');
+    $form['filter_terms'] = [
+      '#type' => 'entity_autocomplete',
+      '#target_type' => 'taxonomy_term',
+      '#title' => $this->t('Related entities must have this URI to be included.'),
+      '#tags' => TRUE,
+      '#default_value' => array_map(function ($element) {
+        return Term::load($element['target_id']);
+      }, array_values($configuration['filter_terms'])),
+      '#required' => TRUE,
+    ];
+    $form['require_all'] = [
+      '#type' => 'checkbox',
+      '#title' => 'Require all terms',
+      '#description' => 'Only index related entities that have all the listed terms.',
+      '#default_value' => $configuration['require_all'],
+    ];
+    // Store the target type of the field, it's a pain to get it when indexing.
+    $form['target_type'] = [
+      '#type' => 'hidden',
+      '#value' => $field->getDataDefinition()->getSetting('target_type'),
+    ];
+    return $form;
+  }
+
+}