Issue #3125467: Drupal 9 readiness

composer.json

@@ -10,5 +10,9 @@
     "issues": "https://www.drupal.org/project/issues/migrate_source_ui",
     "source": "http://cgit.drupalcode.org/migrate_source_ui"
   },
-  "require": { }
+  "require": { },
+  "suggest": {
+      "drupal/migrate_plus": "Provides JSON and XML sources",
+      "drupal/migrate_source_csv": "Provides CSV source"
+  }
 }

migrate_source_ui.info.yml

@@ -2,10 +2,9 @@ name: Migrate Source UI
 type: module
 description: UI for CSV, XML or JSON sources.
 core: 8.x
+core_version_requirement: ^8 || ^9
 package: Migrate
 configure: migrate_source_ui.form
 
 dependencies:
   - drupal:migrate
-  - drupal:migrate_plus
-  - drupal:migrate_source_csv

src/Form/MigrateSourceUiForm.php

@@ -2,6 +2,7 @@
 
 namespace Drupal\migrate_source_ui\Form;
 
+use Drupal\Core\File\FileSystemInterface;
 use Drupal\Core\Form\FormBase;
 use Drupal\Core\Form\FormStateInterface;
 use Drupal\migrate\Plugin\Migration;
@@ -71,7 +72,7 @@ class MigrateSourceUiForm extends FormBase {
       if ($migrationInstance->getSourcePlugin() instanceof CSV || $migrationInstance->getSourcePlugin() instanceof Json || $migrationInstance->getSourcePlugin() instanceof Xml) {
         $id = $definition['id'];
         $options[$id] = $this->t('%id (supports %file_type)', [
-          '%id' => isset($definition['label']) ? $definition['label'] : $id,
+          '%id' => $definition['label'] ?? $id,
           '%file_type' => $this->getFileExtensionSupported($migrationInstance),
         ]);
       }
@@ -109,7 +110,7 @@ class MigrateSourceUiForm extends FormBase {
     $extension = $this->getFileExtensionSupported($migrationInstance);
 
     $validators = ['file_validate_extensions' => [$extension]];
-    $file = file_save_upload('source_file', $validators, FALSE, 0, FILE_EXISTS_REPLACE);
+    $file = file_save_upload('source_file', $validators, FALSE, 0, FileSystemInterface::EXISTS_REPLACE);
 
     if (isset($file)) {
       // File upload was attempted.
@@ -136,9 +137,9 @@ class MigrateSourceUiForm extends FormBase {
 
     // Reset status.
     $status = $migration->getStatus();
-    if ($status != MigrationInterface::STATUS_IDLE) {
+    if ($status !== MigrationInterface::STATUS_IDLE) {
       $migration->setStatus(MigrationInterface::STATUS_IDLE);
-      drupal_set_message($this->t('Migration @id reset to Idle', ['@id' => $migration_id]), 'warning');
+      $this->messenger()->addWarning($this->t('Migration @id reset to Idle', ['@id' => $migration_id]));
     }
 
     $options = [

tests/src/Functional/LoadTest.php

@@ -12,6 +12,11 @@ use Drupal\Tests\BrowserTestBase;
  */
 class LoadTest extends BrowserTestBase {
 
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
   /**
    * Modules to enable.
    *