Don't calculate the theme_token for ajaxPageState unless necessary

I try to figure out why we need a theme token and not just rely on the passed in theme + \Drupal\Core\Theme\ThemeAccessCheck::access

+++ b/core/modules/system/system.module
@@ -666,7 +666,12 @@ function system_js_settings_alter(&$settings, AttachedAssetsInterface $assets) {
+      if ($theme_key !== $this->configFactory->get('system.theme')->get('default')) {

Quick note: We have an API wrapper for that: \Drupal\Core\Extension\ThemeHandlerInterface::getDefault

+++ b/core/modules/system/system.module
@@ -666,7 +666,12 @@ function system_js_settings_alter(&$settings, AttachedAssetsInterface $assets) {
+      if ($theme_key !== $this->configFactory->get('system.theme')->get('default')) {

so yeah $this does not work in an ordinary function. ... Let's use \Drupal::service('theme_handler')->getDefault() instead.

So fast :)

I think it would be cool to have a quick test to ensure that this variable is not always printed out.

Just a quick assumption, I would assume we don't show it at the moment at all.

Yes, a quick test would be great for:

- normal theme
- seven (admin) theme

Looks great, besides that.

+++ b/core/modules/system/system.module
@@ -689,8 +689,13 @@ function system_js_settings_alter(&$settings, AttachedAssetsInterface $assets) {
+      if (\Drupal::theme()->getActiveTheme()->getName() !== \Drupal::service('theme_handler')->getDefault()) {
...
+          ->get(\Drupal::theme()->getActiveTheme()->getName());

Both of these are getting the theme name. Let's store it in a variable.

I'm not sure that this test is complete. In fact, I don't understand exactly how to make an Ajax request that uses a different theme than the default. Right now the test is using the classy theme and behaves as expected.

You could for example have a different admin theme + have a route on /admin/... which then should generate the theme token.

+1

Added a beta eval

This issue is a major task that will improve performance and the disruption it introduces is limited. Per https://www.drupal.org/core/beta-changes, this is a good change to complete during the Drupal 8 beta phase. Committed 767cec2 and pushed to 8.0.x. Thanks!