WebTest tests should have access to the correct DIC

Why don't we simply save the old container in setUp() and restoring in tearDown()?

The approach of keeping stuff around from the parent environment in the child environment in this patch is very at odd with how we handle the rest of the environment switching in the testing framework. Also, if I read this correctly, you are shutting down the testing kernel in the parent environment, not in the child, which is also really odd (which could lead to stuff like caches being set or flushed in the incorrect environment).

Given the number of things this affects...

Tempted to mark this rtbc...

I'll succumb to temptation. :-) #13 looks good to me. Thanks, Kat!

If I'm not mistaken, we are still shutting down the testing container while back inside the parent environment (with the parent database active, etc.):

     // Rebuild caches.
     $this->refreshVariables();
 
+    // Destroy the testing kernel.
+    if (isset($this->kernel)) {
+      $this->kernel->shutdown();
+    }

This needs to move way higher in the tearDown() sequence.

Going on the logic that tearDown() should done in the reverse order to setUp() I've moved the $this->kernel->shutdown(); to just before the simpletest database tables are deleted.

Also I can confirm that this patch fixes #1719936: Theme API tests and Theme functions tests broken if run in a browser

#1719936: Theme API tests and Theme functions tests broken if run in a browser is a critical and is postponed on this, so bumping this to critical.

+++ b/core/includes/bootstrap.incundefined
@@ -2644,7 +2644,7 @@ function language($type, $reset = FALSE) {
-    if (drupal_container()->has('language_manager')) {
+    if (drupal_container()->isScopeActive('request')) {

So now we are use that if we are in the request scope then language_manager service exists?

+++ b/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.phpundefined
@@ -656,6 +657,12 @@ abstract class WebTestBase extends TestBase {
+    $kernel = new DrupalKernel('testing', FALSE);

I miss the wall of test explaining that while the 'prod' DrupalKernel in index.php has a request scope somehow the 'testing' kernel somehow doesn't and how this affects the DIC and so on.

+++ b/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.phpundefined
@@ -656,6 +657,12 @@ abstract class WebTestBase extends TestBase {
+    $kernel->boot();

The kernel is not booted in index.php. Is it necessary here?

Oh and...

Oh, per #13 it doesnt need tests. Thanks aspilicious for pointing this out. Still the black magic needs commenting.

> the language_manager service is only available within the scope of a request

But is it available always?

> somehow the 'testing' kernel somehow doesn't

This is not explained in your followup nor in code comment. Basically, I am asking why does your patch work??

OK, I get it, thanks to the brief explanation by kat in IRC and a little digging in core. language_manager is registered byt the CoreBundle which also activates the request scope. The rest is explained in the patch. I daresay this is ready: I only added comments.

Tweaked the second paragraph more.

beejeebus tuned it further.

RTBC +1 for the changes in #28

I confess I don't quite follow this patch, but it has +1s from the right people and yay for unblocking testing. :)

Committed and pushed to 8.x. Yay!