Theme must not be primed in core.extension and KernelTestBase

It's not really clear to me what fails without those changes and why they were necessary, so let's start with this.

Briefly discussed with @dawehner in IRC already.

We probably need a custom theme negotiator in the (early) installer to resemble the original behavior of a custom installation profile theme. That should get us past the initial "Drupal installation failed" test failure.

The other expectation for kernel tests likely needs an adjustment to ThemeInitialization service, so as to fall back to the (fake) "core" theme in case no default theme is configured, or no themes are enabled.

Some helpful links:

Installer theme selection in install_begin_request():
http://cgit.drupalcode.org/drupal/tree/core/includes/install.core.inc#n395

The case of "no/unknown theme" was handled by the if (!$theme || !isset($themes[$theme])) condition in drupal_theme_initialize() in:
http://cgit.drupalcode.org/drupal/diff/core/includes/theme.inc?id=a4125e...

Attached patch allows me to manually install locally. Let's see what the bot has to say.

+++ b/core/lib/Drupal/Core/Theme/ThemeInitialization.php
@@ -61,29 +61,32 @@ public function getActiveThemeByName($theme_name) {
-      if (empty($themes)) {
-        throw new \RuntimeException('No theme is enabled.');
-      }
-      if (!isset($themes[$theme_name])) {
-        throw new \InvalidArgumentException(String::format('Theme %theme is not enabled/does not exist.', array('theme' => $theme_name)));
-      }
...
+    // If no theme could be negotiated, or if the negotiated theme is not within
+    // the list of enabled themes, fall back to the default theme output of core
+    // and modules (similar to Stark, but without a theme extension at all). This
+    // is possible, because loadActiveTheme() always loads the Twig theme engine.
+    if (empty($themes) || !$theme_name || !isset($themes[$theme_name])) {
+      $theme_name = 'core';
+      // /core/core.info.yml does not actually exist, but is required because
+      // Extension expects a pathname.
+      $active_theme = $this->getActiveTheme(new Extension('theme', 'core/core.info.yml'));
+      return $active_theme;
+    }

In essence, this just simply resurrects the original code from http://cgit.drupalcode.org/drupal/diff/core/includes/theme.inc?id=a4125e...

1. Addresses #5.
2. Reverts additional changes/workarounds from #2228093: Modernize theme initialization that are caused by the same root cause.
3. Adds a regression test to KernelTestBaseTest, which would have prevented at least the KTB::setUp() change, but didn't exist yet, so it's no surprise that the unintentional change in behavior wasn't caught :)

The installer still works as before and as expected, because neither _drupal_maintenance_theme() nor install_begin_request() was changed by #2228093: Modernize theme initialization

The expected behavior is covered by the DistributionProfileTest already.

Updating issue summary accordingly.

That said, while the KTB expectations can be tested, the expected emptiness of the core.extension.yml default config cannot.

That is, unless we'd write a relatively dumb test that would do nothing else than to load the default config file and assert that it's empty.

Sounds a little wonky, but might actually be worth to add.

Added clean regression unit test for core.extension.yml + slightly adjusted docs in ThemeInitialization.

Last patch should still be green, adds regression test coverage, and thus should be ready to be committed.

Clarified the issue summary.

@dawehner: I considered that, but (1) it's unlikely to change any time soon, and (2) in case a new property would be added, then this test should cover that new property, too, but individual assertions would miss the addition, so the new property would be able to prime an extension ahead of time. The test should intentionally fail in case of any change.