diff -u b/core/includes/module.inc b/core/includes/module.inc
--- b/core/includes/module.inc
+++ b/core/includes/module.inc
@@ -79,13 +79,13 @@
   // Use the advanced drupal_static() pattern, since this is called very often.
   static $drupal_static_fast;
   if (!isset($drupal_static_fast)) {
-    $drupal_static_fast['list'] = &drupal_static(__FUNCTION__ . ':list', array());
+    $drupal_static_fast['list'] = &drupal_static(__FUNCTION__ . ':list');
     $drupal_static_fast['sorted_list'] = &drupal_static(__FUNCTION__ . ':sorted_list');
   }
   $list = &$drupal_static_fast['list'];
   $sorted_list = &$drupal_static_fast['sorted_list'];
 
-  if (empty($list) || $refresh || isset($fixed_list)) {
+  if (!isset($list) || $refresh || isset($fixed_list)) {
     $list = array();
     $sorted_list = NULL;
     // The fixed list may be a completely empty array, thus check for isset().
diff -u b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
--- b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
@@ -701,6 +701,9 @@
     Database::removeConnection('default');
     Database::renameConnection('simpletest_original_default', 'default');
 
+    // Reset all static variables.
+    drupal_static_reset();
+
     // Restore original in-memory configuration.
     $conf = $this->originalConf;
 
diff -u b/core/modules/simpletest/lib/Drupal/simpletest/UnitTestBase.php b/core/modules/simpletest/lib/Drupal/simpletest/UnitTestBase.php
--- b/core/modules/simpletest/lib/Drupal/simpletest/UnitTestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/UnitTestBase.php
@@ -43,17 +43,24 @@
 
     // Prepare the environment for running tests.
     $this->prepareEnvironment();
-    $this->originalModuleList = module_list();
-
-    // Empty out module list.
-    module_list(TRUE, FALSE, FALSE, array());
-    module_load_all();
-    module_implements_reset();
+    $this->originalThemeRegistry = theme_get_registry(FALSE);
 
     // Reset all statics and variables to perform tests in a clean environment.
     $conf = array();
     drupal_static_reset();
 
+    // Empty out module list.
+    module_list(TRUE, FALSE, FALSE, array());
+    // Prevent module_load_all() from attempting to refresh it.
+    $has_run = &drupal_static('module_load_all');
+    $has_run = TRUE;
+
+    // Re-implant theme registry.
+    // Required to l() and other functions to work correctly and not trigger
+    // database lookups.
+    $theme_get_registry = &drupal_static('theme_get_registry');
+    $theme_get_registry[FALSE] = $this->originalThemeRegistry;
+
     $conf['file_public_path'] = $this->public_files_directory;
 
     // Change the database prefix.
@@ -69,11 +76,2 @@
   }
-
-  protected function tearDown() {
-    parent::tearDown();
-
-    // Restore module list.
-    module_list(TRUE, FALSE, FALSE, $this->originalModuleList);
-    module_load_all();
-    module_implements_reset();
-  }
 }