diff --git a/core/modules/system/tests/themes/test_theme/css/collapse.css b/core/modules/system/tests/themes/test_theme/css/collapse.css
new file mode 100644
index 0000000..a774ab4
--- /dev/null
+++ b/core/modules/system/tests/themes/test_theme/css/collapse.css
@@ -0,0 +1,4 @@
+/**
+ * @file
+ * Test css asset file for test_theme.theme
+ */
diff --git a/core/modules/system/tests/themes/test_theme/js/collapse.js b/core/modules/system/tests/themes/test_theme/js/collapse.js
new file mode 100644
index 0000000..05e01dd
--- /dev/null
+++ b/core/modules/system/tests/themes/test_theme/js/collapse.js
@@ -0,0 +1,4 @@
+/**
+ * @file
+ * Test js asset file for test_theme.theme
+ */
diff --git a/core/modules/system/tests/themes/test_theme/test_theme.info.yml b/core/modules/system/tests/themes/test_theme/test_theme.info.yml
index d776107..d81c4e3 100644
--- a/core/modules/system/tests/themes/test_theme/test_theme.info.yml
+++ b/core/modules/system/tests/themes/test_theme/test_theme.info.yml
@@ -23,10 +23,10 @@ regions:
 
 libraries-override:
   # Replace an entire library.
-  'core/drupal.collapse': 'test_theme/collapse'
+  core/drupal.collapse: 'test_theme/collapse'
   # Remove an entire library.
-  'core/drupal.progress': FALSE
+  core/drupal.progress: FALSE
   # Replace one particular library asset with another.
-  'classy/base/css/theme/css/layout.css': 'css/test_theme_layout.css'
+  classy/base/css/theme/css/layout.css: 'css/test_theme_layout.css'
   # Remove one particular asset.
-  'core/drupal.dialog/css/theme/misc/dialog.theme.css': FALSE
+  core/drupal.dialog/css/theme/misc/dialog.theme.css: FALSE