diff -u b/core/modules/field/tests/src/Kernel/Migrate/d6/MigrateFieldInstanceTest.php b/core/modules/field/tests/src/Kernel/Migrate/d6/MigrateFieldInstanceTest.php
--- b/core/modules/field/tests/src/Kernel/Migrate/d6/MigrateFieldInstanceTest.php
+++ b/core/modules/field/tests/src/Kernel/Migrate/d6/MigrateFieldInstanceTest.php
@@ -104,6 +104,7 @@
 
     // Test date field.
     $field = FieldConfig::load('node.story.field_test_date');
+    $this->assertInstanceOf(FieldConfig::class, $field);
     $this->assertSame('Date Field', $field->label());
     $this->assertSame('An example date field.', $field->getDescription());
     $expected = ['datetime_type' => 'datetime'];
@@ -119,6 +120,7 @@
 
     // Test datetime field.
     $field = FieldConfig::load('node.story.field_test_datetime');
+    $this->assertInstanceOf(FieldConfig::class, $field);
     $this->assertSame('Datetime Field', $field->label());
     $this->assertSame('An example datetime field.', $field->getDescription());
     $expected = ['datetime_type' => 'datetime'];
@@ -129,6 +131,7 @@
 
     // Test datestamp field.
     $field = FieldConfig::load('node.story.field_test_datestamp');
+    $this->assertInstanceOf(FieldConfig::class, $field);
     $this->assertSame('Date Stamp Field', $field->label());
     $this->assertSame('An example date stamp field.', $field->getDescription());
     $expected = [];
diff -u b/core/modules/field/tests/src/Kernel/Migrate/d6/MigrateFieldTest.php b/core/modules/field/tests/src/Kernel/Migrate/d6/MigrateFieldTest.php
--- b/core/modules/field/tests/src/Kernel/Migrate/d6/MigrateFieldTest.php
+++ b/core/modules/field/tests/src/Kernel/Migrate/d6/MigrateFieldTest.php
@@ -64,11 +64,11 @@
 
     // Date fields.
     $field_storage = FieldStorageConfig::load('node.field_test_datetime');
-    $this->assertSame("datetime", $field_storage->getType(), t('Field type is @fieldtype. It should be datetime.', array('@fieldtype' => $field_storage->getType())));
+    $this->assertSame("datetime", $field_storage->getType(), t('Field type is @fieldtype. It should be datetime.', ['@fieldtype' => $field_storage->getType()]));
     $field_storage = FieldStorageConfig::load('node.field_test_datestamp');
-    $this->assertSame("timestamp", $field_storage->getType(), t('Field type is @fieldtype. It should be timestamp.', array('@fieldtype' => $field_storage->getType())));
+    $this->assertSame("timestamp", $field_storage->getType(), t('Field type is @fieldtype. It should be timestamp.', ['@fieldtype' => $field_storage->getType()]));
     $field_storage = FieldStorageConfig::load('node.field_test_date');
-    $this->assertSame("datetime", $field_storage->getType(), t('Field type is @fieldtype. It should be datetime.', array('@fieldtype' => $field_storage->getType())));
+    $this->assertSame("datetime", $field_storage->getType(), t('Field type is @fieldtype. It should be datetime.', ['@fieldtype' => $field_storage->getType()]));
 
     // Decimal field with radio buttons.
     $field_storage = FieldStorageConfig::load('node.field_test_decimal_radio_buttons');