diff --git a/core/lib/Drupal/Core/Config/GlobalConfig.php b/core/lib/Drupal/Core/Config/GlobalConfig.php
index 204697e..9d29ed6 100644
--- a/core/lib/Drupal/Core/Config/GlobalConfig.php
+++ b/core/lib/Drupal/Core/Config/GlobalConfig.php
@@ -86,7 +86,7 @@ public function set($config_name, $keys, $value) {
     // Inlined NestedArray::setValue(), because this class is used before the
     // class loader is available.
     $ref = &$this->storage;
-    foreach (array_merge($config_name, explode('.', $keys)) as $key) {
+    foreach (array_merge(array($config_name), explode('.', $keys)) as $key) {
       $ref = &$ref[$key];
     }
     $ref = $value;
@@ -113,7 +113,7 @@ public function del($config_name, $keys = NULL) {
     else {
       // Inlined NestedArray::unsetValue(), because this class is used before
       // the class loader is available.
-      $parents = array_merge($config_name, explode('.', $keys));
+      $parents = array_merge(array($config_name), explode('.', $keys));
       $unset_key = array_pop($parents);
       $ref = &$this->storage;
       foreach ($parents as $key) {
diff --git a/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
index e8f72a4..d1bb1e3 100644
--- a/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
@@ -229,11 +229,6 @@ public static function getInfo() {
   }
 
   /**
-   * Performs setup tasks before each individual test method is run.
-   */
-  abstract protected function setUp();
-
-  /**
    * Checks the matching requirements for Test.
    *
    * @return
@@ -920,7 +915,6 @@ protected function prepareEnvironment() {
       $this->originalPrefix = drupal_valid_test_ua();
       drupal_valid_test_ua($this->databasePrefix);
     }
-    $this->globalConfig = GlobalConfig::getInstance();
 
     // Backup current in-memory configuration.
     $this->originalSettings = settings()->getAll();
@@ -942,9 +936,6 @@ protected function prepareEnvironment() {
     $this->originalProfile = drupal_get_profile();
     $this->originalUser = isset($user) ? clone $user : NULL;
 
-    // Remove all configuration overrides.
-    $this->globalConfig->setConfig(array());
-
     // Ensure that the current session is not changed by the new environment.
     require_once DRUPAL_ROOT . '/' . settings()->get('session_inc', 'core/includes/session.inc');
     drupal_save_session(FALSE);
@@ -978,12 +969,7 @@ protected function prepareEnvironment() {
     // Create and set new configuration directories.
     $this->prepareConfigDirectories();
 
-    // Apply global configuration override values.
-    foreach ($this->configOverrides as $config_name => $config_values) {
-      foreach ($config_values as $key => $value) {
-        $this->globalConfig->set($config_name, $key, $value);
-      }
-    }
+    $this->globalConfig = GlobalConfig::getInstance();
 
     // Reset statics before the old container is replaced so that objects with a
     // __destruct() method still have access to it.
@@ -1078,6 +1064,45 @@ protected function rebuildContainer() {
   }
 
   /**
+   * Performs setup tasks before each individual test method is run.
+   */
+  protected function setUp() {
+    // Create the database prefix for this test.
+    $this->prepareDatabasePrefix();
+
+    // Prepare the environment for running tests.
+    $this->prepareEnvironment();
+    if (!$this->setupEnvironment) {
+      throw new \RuntimeException('Failed to set up test environment.');
+    }
+
+    // Change the database prefix.
+    // All static variables need to be reset before the database prefix is
+    // changed, since \Drupal\Core\Utility\CacheArray implementations attempt to
+    // write back to persistent caches when they are destructed.
+    $this->changeDatabasePrefix();
+    if (!$this->setupDatabasePrefix) {
+      throw new \RuntimeException('Failed to set up database prefix.');
+    }
+
+    // After preparing the environment and changing the database prefix, we are
+    // in a valid test environment.
+    drupal_valid_test_ua($this->databasePrefix);
+
+    // Remove all configuration overrides.
+    $this->globalConfig->setConfig(array());
+
+    // Apply global configuration override values.
+    foreach ($this->configOverrides as $config_name => $config_values) {
+      foreach ($config_values as $key => $value) {
+        $this->globalConfig->set($config_name, $key, $value);
+      }
+    }
+
+    $this->setup = TRUE;
+  }
+
+  /**
    * Performs cleanup tasks after each individual test method has been run.
    *
    * Deletes created files, database tables, and reverts environment changes.
diff --git a/core/modules/simpletest/lib/Drupal/simpletest/UnitTestBase.php b/core/modules/simpletest/lib/Drupal/simpletest/UnitTestBase.php
index f2bad12..7f20fed 100644
--- a/core/modules/simpletest/lib/Drupal/simpletest/UnitTestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/UnitTestBase.php
@@ -41,29 +41,8 @@ function __construct($test_id = NULL) {
    * setUp() method.
    */
   protected function setUp() {
-    // Create the database prefix for this test.
-    $this->prepareDatabasePrefix();
-
-    // Prepare the environment for running tests.
-    $this->prepareEnvironment();
-    if (!$this->setupEnvironment) {
-      return FALSE;
-    }
-
-    // Reset all statics to perform tests in a clean environment.
-    drupal_static_reset();
+    parent::setUp();
 
     $this->settingsSet('file_public_path', $this->public_files_directory);
-
-    // Change the database prefix.
-    // All static variables need to be reset before the database prefix is
-    // changed, since \Drupal\Core\Utility\CacheArray implementations attempt to
-    // write back to persistent caches when they are destructed.
-    $this->changeDatabasePrefix();
-    if (!$this->setupDatabasePrefix) {
-      return FALSE;
-    }
-
-    $this->setup = TRUE;
   }
 }
diff --git a/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php b/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
index ef2d396..90bb51b 100644
--- a/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
@@ -724,26 +724,7 @@ protected function setUp() {
     // Backup the currently running Simpletest batch.
     $this->originalBatch = batch_get();
 
-    // Create the database prefix for this test.
-    $this->prepareDatabasePrefix();
-
-    // Prepare the environment for running tests.
-    $this->prepareEnvironment();
-    if (!$this->setupEnvironment) {
-      return FALSE;
-    }
-
-    // Reset all statics to perform tests in a clean environment.
-    drupal_static_reset();
-
-    // Change the database prefix.
-    // All static variables need to be reset before the database prefix is
-    // changed, since \Drupal\Core\Utility\CacheArray implementations attempt to
-    // write back to persistent caches when they are destructed.
-    $this->changeDatabasePrefix();
-    if (!$this->setupDatabasePrefix) {
-      return FALSE;
-    }
+    parent::setUp();
 
     // Add the parent profile's search path to the child site's search paths.
     // @see drupal_system_listing()
diff --git a/core/modules/system/lib/Drupal/system/Tests/InstallerTest.php b/core/modules/system/lib/Drupal/system/Tests/InstallerTest.php
index 6edabe0..2ae1f78 100644
--- a/core/modules/system/lib/Drupal/system/Tests/InstallerTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/InstallerTest.php
@@ -37,21 +37,25 @@ protected function setUp() {
     // Prepare the environment for running tests.
     $this->prepareEnvironment();
     if (!$this->setupEnvironment) {
-      return FALSE;
+      throw new \RuntimeException('Failed to set up test environment.');
     }
 
-    // Reset all statics to perform tests in a clean environment.
-    drupal_static_reset();
-
     // Change the database prefix.
     // All static variables need to be reset before the database prefix is
     // changed, since \Drupal\Core\Utility\CacheArray implementations attempt to
     // write back to persistent caches when they are destructed.
     $this->changeDatabasePrefix();
     if (!$this->setupDatabasePrefix) {
-      return FALSE;
+      throw new \RuntimeException('Failed to set up database prefix.');
     }
 
+    // After preparing the environment and changing the database prefix, we are
+    // in a valid test environment.
+    drupal_valid_test_ua($this->databasePrefix);
+
+    // Remove all configuration overrides.
+    $this->globalConfig->setConfig(array());
+
     $this->globalConfig->set('system.file', 'path.private', $this->private_files_directory);
     $this->globalConfig->set('system.file', 'path.temporary', $this->temp_files_directory);
     $this->globalConfig->set('locale.settings', 'translation.path', $this->translation_files_directory);