Issue #3029627 by bobbygryzynger, tim.plunkett, xjm, johndevman: FormatterBase...

        '#items' => $items,

        '#formatter' => $this->getPluginId(),

        '#is_multiple' => $this->fieldDefinition->getFieldStorageDefinition()->isMultiple(),

        '#third_party_settings' => $this->getThirdPartySettings(),

      ];

      $elements = array_merge($info, $elements);

<?php

namespace Drupal\Tests\field\Kernel;

use Drupal\Core\Entity\Entity\EntityViewDisplay;

use Drupal\entity_test\Entity\EntityTestRev;

use Drupal\field\Entity\FieldConfig;

use Drupal\field\Entity\FieldStorageConfig;

use Drupal\KernelTests\KernelTestBase;

/**

 * Tests for field formatters.

 *

 * @group field

 */

class FieldFormatterTest extends KernelTestBase {

  /**

   * {@inheritdoc}

   */

  protected static $modules = [

    'field',

    'text',

    'entity_test',

    'field_test',

    'system',

    'filter',

    'user',

  ];

  /**

   * The field's name.

   *

   * @var string

   */

  protected $fieldName;

  /**

   * The default display.

   *

   * @var \Drupal\Core\Entity\Display\EntityViewDisplayInterface

   */

  protected $display;

  /**

   * {@inheritdoc}

   */

  protected function setUp() {

    parent::setUp();

    // Configure the theme system.

    $this->installConfig(['system', 'field']);

    $this->installEntitySchema('entity_test_rev');

    $entity_type = 'entity_test_rev';

    $bundle = $entity_type;

    $this->fieldName = mb_strtolower($this->randomMachineName());

    $field_storage = FieldStorageConfig::create([

      'field_name' => $this->fieldName,

      'entity_type' => $entity_type,

      'type' => 'string',

    ]);

    $field_storage->save();

    $instance = FieldConfig::create([

      'field_storage' => $field_storage,

      'bundle' => $bundle,

      'label' => $this->randomMachineName(),

    ]);

    $instance->save();

    $this->display = EntityViewDisplay::create([

      'targetEntityType' => $entity_type,

      'bundle' => $bundle,

      'mode' => 'default',

      'status' => TRUE,

    ])->setComponent($this->fieldName, [

      'type' => 'string',

      'settings' => [],

    ]);

    $this->display->save();

  }

  /**

   * Tests availability of third party settings in field render arrays.

   */

  public function testThirdPartySettings() {

    $third_party_settings = [

      'field_test' => [

        'foo' => 'bar',

      ],

    ];

    $component = $this->display->getComponent($this->fieldName);

    $component['third_party_settings'] = $third_party_settings;

    $this->display->setComponent($this->fieldName, $component)->save();

    $entity = EntityTestRev::create([]);

    $entity->{$this->fieldName}->value = $this->randomString();

    $build = $entity->{$this->fieldName}->view('default');

    $this->assertEquals($third_party_settings, $build['#third_party_settings']);

  }

}