Remove Classy as a base theme of core themes

1. +++ b/core/modules/block/tests/modules/block_test/config/install/block.block.test_block.yml
   @@ -13,5 +13,5 @@ dependencies:
      theme:
   -    - classy
   +    - stark
   

   This change is either out of scope, or the same change needs to be made to a couple other config files as well.

2. +++ /dev/null
   @@ -1,447 +0,0 @@
   -class ThemeNotUsingClassyLibraryTest extends KernelTestBase {
   

   This is removed, but not ThemesNotUsingClassyTemplatesTest. Just checking if that's correct.

3. +++ b/core/themes/seven/seven.info.yml
   --- a/core/themes/seven/seven.libraries.yml
   +++ b/core/themes/seven/seven.libraries.yml
   
   +++ b/core/themes/seven/seven.libraries.yml
   @@ -6,6 +6,28 @@ global-styling:
        component:
   +      css/classy/components/action-links.css: {}
   

   This seemed confusing/wrong at first, but now I see that `css/classy` is a directory *within* Seven, not that this is referring directly to Classy itself. So, not wrong, just confusing :D

4. I count 10 other test themes that still have base theme: classy
   Is there a follow-up for those, or should they be handled here?

Tests depending on Classy are covered by this meta-issue #3083275: [meta] Update tests that rely on Classy to not rely on it anymore (the title of the issue should probably be updated to make it clear it covers all Classy dependencies in tests, not just ones depending on Classy markup)

Almost half the of the 59k is removing tests that aren't necessary after core themes stop declaring Classy as a base theme.

Went through the remaining code and noted the following:

• Test fixes are straightforward and reflect that fact that core themes are specifying a different base theme
• Removing image-widget and the assets where unnecessary (👌). It was never loaded by Classy - it was just available for themes that subthemed Classy - so it isn't needed by any core theme that doesn't actively use it.
• Reviewed each removed library in libraries-override and confirmed the library would still load either from declaring it in libraries: or it is requested by a template via attach_library. Also confirmed that any files previously loaded by an override of classy/base were now included in the theme's global-styling.

Cross-post, was just going to RTBC as well. #10 and #11 addressed my concerns. Thanks!

1. +++ b/core/modules/system/system.install
   @@ -1271,3 +1272,18 @@ function system_update_9000() {
   +/**
   + * Uninstall Classy if it is no longer needed.
   + */
   +function system_update_9001() {
   +  /** @var \Drupal\Core\Extension\ThemeInstallerInterface $theme_installer */
   +  $theme_installer = \Drupal::getContainer()->get('theme_installer');
   +  try {
   +    $theme_installer->uninstall(['classy']);
   +  }
   +  catch (\InvalidArgumentException | UnknownExtensionException $exception) {
   +    // Exception is thrown if Classy wasn't installed or if there are themes
   +    // depending on it.
   +  }
   +}
   

   @catch has been avoiding a hook_update_N because it backs us into a corner with supporting 8.9.x and 9.0.x at the same time. I think this can be a post update. They don't have the same issues with numbering.

2. +++ b/core/modules/system/tests/src/Functional/Update/ClassyUninstallUpdateTest.php
   @@ -0,0 +1,87 @@
   +/**
   + * Ensures that update hook uninstalls Classy when it's no longer needed.
   + *
   + * @group Update
   + */
   +class ClassyUninstallUpdateTest extends UpdatePathTestBase {
   

   <3 - really great to a comprehensive test.

+++ b/core/modules/system/system.post_update.php
@@ -316,3 +317,18 @@ function system_post_update_entity_revision_metadata_bc_cleanup() {
+  catch (\InvalidArgumentException | UnknownExtensionException $exception) {
+    // Exception is thrown if Classy wasn't installed or if there are themes
+    // depending on it.
+  }

This also stops it being uninstalled if classy if the admin or default theme. Which imo is a nice outcome.

#15 successfully moves the Classy uninstall from hook_update_N to a post update. Switching back to RTBC.

Committed 458e132 and pushed to 9.0.x. Thanks!