diff --git a/core/includes/bootstrap.inc b/core/includes/bootstrap.inc
index ca45730..3f2f591 100644
--- a/core/includes/bootstrap.inc
+++ b/core/includes/bootstrap.inc
@@ -1453,7 +1453,7 @@ function t($string, array $args = array(), array $options = array()) {
   $locale_exists = &drupal_static(__FUNCTION__);
 
   if (!isset($locale_exits)) {
-    $locale_exists = module_exists('locale');
+    $locale_exists = function_exists('module_exists') && module_exists('locale');
   }
 
   // Merge in default.
diff --git a/core/includes/module.inc b/core/includes/module.inc
index a772436..db1947d 100644
--- a/core/includes/module.inc
+++ b/core/includes/module.inc
@@ -11,26 +11,30 @@
 /**
  * Loads all the modules that have been enabled in the system table.
  *
- * @param $bootstrap
+ * @param bool $bootstrap
  *   Whether to load only the reduced set of modules loaded in "bootstrap mode"
- *   for cached pages. See bootstrap.inc.
+ *   for cached pages. See bootstrap.inc. Pass NULL to only check the current
+ *   status without loading of modules.
+ * @param bool $reset
+ *   (optional) Internal use only. Whether to reset the internal statically
+ *   cached flag of whether modules have been loaded. If TRUE, all modules are
+ *   (re)loaded in the same call. Used by the testing framework to override and
+ *   persist a limited module list for the duration of a unit test (in which no
+ *   module system exists).
  *
- * @return
- *   If $bootstrap is NULL, return a boolean indicating whether all modules
- *   have been loaded.
+ * @return bool
+ *   A Boolean indicating whether all modules have been loaded. This means all
+ *   modules; the load status of bootstrap modules cannot be checked.
  */
-function module_load_all($bootstrap = FALSE) {
-  // Already loaded code cannot be unloaded, but new modules may be added within
-  // a request, which should be loaded as well.
-  // Use the advanced drupal_static() pattern, since this is called very often.
-  // @see theme()
-  static $drupal_static_fast;
-  if (!isset($drupal_static_fast)) {
-    $drupal_static_fast['has_run'] = &drupal_static(__FUNCTION__, FALSE);
+function module_load_all($bootstrap = FALSE, $reset = FALSE) {
+  static $has_run = FALSE;
+
+  if ($reset) {
+    $has_run = FALSE;
   }
-  $has_run = &$drupal_static_fast['has_run'];
 
-  if (isset($bootstrap)) {
+  // Unless $boostrap is NULL, load the requested set of modules.
+  if (isset($bootstrap) && !$has_run) {
     $type = $bootstrap ? 'bootstrap' : 'module_enabled';
     foreach (module_list($type) as $module) {
       drupal_load('module', $module);
diff --git a/core/modules/field/modules/field_sql_storage/field_sql_storage.install b/core/modules/field/modules/field_sql_storage/field_sql_storage.install
index 2229ef4..0bbdc5a 100644
--- a/core/modules/field/modules/field_sql_storage/field_sql_storage.install
+++ b/core/modules/field/modules/field_sql_storage/field_sql_storage.install
@@ -111,6 +111,9 @@ function field_sql_storage_update_8000(&$sandbox) {
   // Retrieve field data.
   $fields = _update_7000_field_read_fields(array('storage_type' => 'field_sql_storage'));
 
+  // Load private table name helper functions in field_sql_storage.module.
+  drupal_load('module', 'field_sql_storage');
+
   // Update schema.
   foreach ($fields as $field) {
     $data_table = _field_sql_storage_tablename($field);
diff --git a/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
index 00eb07f..048da36 100644
--- a/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
@@ -796,8 +796,9 @@ protected function tearDown() {
     // Reset all static variables.
     drupal_static_reset();
 
-    // Reset module list.
+    // Reset module list and module load status.
     module_list_reset();
+    module_load_all(FALSE, TRUE);
 
     // Restore original in-memory configuration.
     $conf = $this->originalConf;
diff --git a/core/modules/system/lib/Drupal/system/Tests/Upgrade/UpgradePathTestBase.php b/core/modules/system/lib/Drupal/system/Tests/Upgrade/UpgradePathTestBase.php
index a0f6c59..86456e5 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Upgrade/UpgradePathTestBase.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Upgrade/UpgradePathTestBase.php
@@ -191,13 +191,13 @@ protected function performUpgrade($register_errors = TRUE) {
     $update_url = $GLOBALS['base_url'] . '/core/update.php';
     $this->drupalGet($update_url, array('external' => TRUE));
     if (!$this->assertResponse(200)) {
-      return FALSE;
+      throw new Exception('Initial GET to update.php did not return HTTP 200 status.');
     }
 
     // Continue.
     $this->drupalPost(NULL, array(), t('Continue'));
     if (!$this->assertResponse(200)) {
-      return FALSE;
+      throw new Exception('POST to continue update.php did not return HTTP 200 status.');
     }
 
     // The test should pass if there are no pending updates.
@@ -211,7 +211,7 @@ protected function performUpgrade($register_errors = TRUE) {
     // Go!
     $this->drupalPost(NULL, array(), t('Apply pending updates'));
     if (!$this->assertResponse(200)) {
-      return FALSE;
+      throw new Exception('POST to update.php to apply pending updates did not return HTTP 200 status.');
     }
 
     // Check for errors during the update process.
@@ -222,36 +222,30 @@ protected function performUpgrade($register_errors = TRUE) {
         $this->fail($message);
       }
     }
-
     if (!empty($this->upgradeErrors)) {
       // Upgrade failed, the installation might be in an inconsistent state,
       // don't process.
-      return FALSE;
+      throw new Exception('Errors during update process.');
     }
 
     // Check if there still are pending updates.
     $this->drupalGet($update_url, array('external' => TRUE));
     $this->drupalPost(NULL, array(), t('Continue'));
     if (!$this->assertText(t('No pending updates.'), t('No pending updates at the end of the update process.'))) {
-      return FALSE;
+      throw new Exception('update.php still shows pending updates after execution.');
     }
 
     // Upgrade succeed, rebuild the environment so that we can call the API
     // of the child site directly from this request.
     $this->upgradedSite = TRUE;
 
-    // Reload module list. For modules that are enabled in the test database,
-    // but not on the test client, we need to load the code here.
+    // Reload module list for modules that are enabled in the test database
+    // but not on the test client.
     system_list_reset();
-    foreach (module_list() as $module) {
-      drupal_load('module', $module);
-    }
-
-    // Reload hook implementations
     module_implements_reset();
+    module_load_all(FALSE, TRUE);
 
     // Rebuild caches.
-    drupal_static_reset();
     drupal_flush_all_caches();
 
     // Reload global $conf array and permissions.
diff --git a/core/modules/user/user.install b/core/modules/user/user.install
index 67957da..a22b2d8 100644
--- a/core/modules/user/user.install
+++ b/core/modules/user/user.install
@@ -351,11 +351,11 @@ function user_install() {
  * The 'Member for' extra field has moved one level up in the array.
  */
 function user_update_8000() {
-  $settings = field_bundle_settings('user', 'user');
+  $settings = update_variable_get('field_bundle_settings_user__user', array());
   if (isset($settings['extra_fields']['display']['summary'])) {
     $settings['extra_fields']['display']['member_for'] = $settings['extra_fields']['display']['summary'];
     unset($settings['extra_fields']['display']['summary']);
-    field_bundle_settings('user', 'user', $settings);
+    update_variable_set('field_bundle_settings_user__user', $settings);
   }
 }
 
diff --git a/core/update.php b/core/update.php
index b8a726d..1a00777 100644
--- a/core/update.php
+++ b/core/update.php
@@ -366,10 +366,6 @@ function update_check_requirements($skip_warnings = FALSE) {
   }
 }
 
-// Some unavoidable errors happen because the database is not yet up-to-date.
-// Our custom error handler is not yet installed, so we just suppress them.
-ini_set('display_errors', FALSE);
-
 // We prepare a minimal bootstrap for the update requirements check to avoid
 // reaching the PHP memory limit.
 require_once DRUPAL_ROOT . '/core/includes/bootstrap.inc';
@@ -378,6 +374,12 @@ function update_check_requirements($skip_warnings = FALSE) {
 require_once DRUPAL_ROOT . '/core/includes/file.inc';
 require_once DRUPAL_ROOT . '/core/includes/unicode.inc';
 require_once DRUPAL_ROOT . '/core/includes/schema.inc';
+
+// Limit the module list to System module for the entire duration of update.php.
+include_once DRUPAL_ROOT . '/core/includes/module.inc';
+$module_list['system']['filename'] = 'core/modules/system/system.module';
+module_list(NULL, $module_list);
+
 update_prepare_d8_bootstrap();
 
 // Determine if the current user has access to run update.php.
@@ -404,9 +406,6 @@ function update_check_requirements($skip_warnings = FALSE) {
   require_once DRUPAL_ROOT . '/core/modules/system/system.install';
 
   // Load module basics.
-  include_once DRUPAL_ROOT . '/core/includes/module.inc';
-  $module_list['system']['filename'] = 'core/modules/system/system.module';
-  module_list(NULL, $module_list);
   drupal_load('module', 'system');
 
   // Reset the module_implements() cache so that any new hook implementations