Make Classy extend from the new Stable base theme

We can maybe make Classy extend from Stable in this issue as well. Let's see how it goes, we can split later if needed.

I think we need something better than this because as soon as someone calls something like getActiveTheme() on ThemeManager or ThemeInitialization during updates things will probably go wrong. But here is something @lauriii and myself have been kicking around, he did the fancy things.

Originally I wanted to just put this stuff right into ThemeInitialization but I guess that's not good.

@lauriii mentioned in the frontend meeting today maybe doing something that's hanging off of UpdateKernel, that seemed sensible to me.

Just want to make sure we don't forget this interdiff, moving from #2575737-21: Copy templates, CSS, and related assets to Stable and override core libraries' CSS with an additional change I caught. Any themes extending from Classy or Stable should not reference core libraries/templates/assets/etc. because they may change.

Although that type of change may need to be follow-up to both of these issues unless #2575737: Copy templates, CSS, and related assets to Stable and override core libraries' CSS gets in first. So maybe roll that back for now because it will make tests fail.

Thanks @lauriii!

1. +++ b/core/includes/theme.maintenance.inc
   @@ -6,6 +6,7 @@
   +use Drupal\Core\Theme\MissingThemeDependencyException;
   

   Minor: I think this use is no longer needed.

2. +++ b/core/lib/Drupal/Core/Theme/MissingThemeDependencyException.php
   @@ -0,0 +1,37 @@
   + * @see \Drupal\Core\Theme\ThemeInitialization::getActiveThemeByName().
   

   Update @see to point to the relevant spot.

3. +++ b/core/modules/system/src/Controller/DbUpdateController.php
   @@ -101,8 +126,12 @@ class DbUpdateController extends ControllerBase {
   +   * @param \Drupal\Core\Theme\ThemeInitializationInterface $theme_initialization
   +   *   The theme initialization.
   +   * @param \Drupal\Core\Extension\ThemeHandlerInterface $theme_handler
   +   *   The theme handler.
   ...
   -  public function __construct($root, KeyValueExpirableFactoryInterface $key_value_expirable_factory, CacheBackendInterface $cache, StateInterface $state, ModuleHandlerInterface $module_handler, AccountInterface $account, BareHtmlPageRendererInterface $bare_html_page_renderer, UpdateRegistry $post_update_registry) {
   +  public function __construct($root, KeyValueExpirableFactoryInterface $key_value_expirable_factory, CacheBackendInterface $cache, StateInterface $state, ModuleHandlerInterface $module_handler, AccountInterface $account, BareHtmlPageRendererInterface $bare_html_page_renderer, UpdateRegistry $post_update_registry, ThemeInitializationInterface $theme_initialization, ThemeHandlerInterface $theme_handler, ThemeInstallerInterface $theme_installer) {
   
   @@ -111,6 +140,9 @@ public function __construct($root, KeyValueExpirableFactoryInterface $key_value_
   +    $this->themeInitialization = $theme_initialization;
   +    $this->themeHandler = $theme_handler;
   +    $this->themeInstaller = $theme_installer;
   

   Minor: Missing @param for $theme_installer.

How can we test this code?

Created #2588289: Update Classy's Twig extends now that it extends from Stable for things like #10.

Manual testing seems to check out, bot is happy, and this approach seems sensible to me. I could only find these minor points on review:

1. +++ b/core/lib/Drupal/Core/Theme/MissingThemeDependencyException.php
   @@ -0,0 +1,48 @@
   + * @see \Drupal\system\Controller\DbUpdateController::ensureActiveThemeDependencies().
   

   Minor: I think this line can be removed now since we don't throw the exception there any more.

2. +++ b/core/lib/Drupal/Core/Theme/ThemeInitialization.php
   @@ -109,6 +109,16 @@ public function getActiveThemeByName($theme_name) {
   +          // regressions on the update.php
   

   Minor: Missing period at the end.

Just a reroll after #2574975: Allow (base-)themes to be excluded from the UI (e.g. blocks, Appearance). got committed, no changes.

Okay so the only things I can see that need work are #33 (and removing the reference in that chunk of docs to system_update_8012() which is being removed now).

Since it's docs only, rolling in here and RTBC'ing – based on my previous reviews and @lauriii's in #34. I think @alexpott, @lauriii, and me are all happy with this now :)