diff --git a/core/lib/Drupal/Core/Asset/CssCollectionOptimizer.php b/core/lib/Drupal/Core/Asset/CssCollectionOptimizer.php
index 7a10af0..5bc4a7d 100644
--- a/core/lib/Drupal/Core/Asset/CssCollectionOptimizer.php
+++ b/core/lib/Drupal/Core/Asset/CssCollectionOptimizer.php
@@ -8,6 +8,7 @@
 
 use Drupal\Core\Asset\AssetCollectionOptimizerInterface;
 use Drupal\Core\KeyValueStore\KeyValueStoreInterface;
+use Drupal\Core\KeyValueStore\StateInterface;
 
 /**
  * Optimizes CSS assets.
@@ -38,7 +39,7 @@ class CssCollectionOptimizer implements AssetCollectionOptimizerInterface {
   /**
    * The state key/value store.
    *
-   * @var \Drupal\Core\KeyValueStore\KeyValueStoreInterface
+   * @var \Drupal\Core\KeyValueStore\StateInterface
    */
   protected $state;
 
@@ -51,7 +52,7 @@ class CssCollectionOptimizer implements AssetCollectionOptimizerInterface {
    *   The optimizer for a single CSS asset.
    * @param \Drupal\Core\Asset\AssetDumperInterface
    *   The dumper for optimized CSS assets.
-   * @param \Drupal\Core\KeyValueStore\KeyValueStoreInterface
+   * @param \Drupal\Core\KeyValueStore\StateInterface
    *   The state key/value store.
    */
   public function __construct(AssetCollectionGrouperInterface $grouper, AssetOptimizerInterface $optimizer, AssetDumperInterface $dumper, StateInterface $state) {
diff --git a/core/lib/Drupal/Core/KeyValueStore/State.php b/core/lib/Drupal/Core/KeyValueStore/State.php
index 75b950e..57b96dc 100644
--- a/core/lib/Drupal/Core/KeyValueStore/State.php
+++ b/core/lib/Drupal/Core/KeyValueStore/State.php
@@ -113,4 +113,11 @@ public function delete($key) {
     $this->deleteMultiple(array($key));
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  public function resetCache() {
+    $this->cache = array();
+  }
+
 }
diff --git a/core/lib/Drupal/Core/KeyValueStore/StateInterface.php b/core/lib/Drupal/Core/KeyValueStore/StateInterface.php
index 36f705c..9e248d8 100644
--- a/core/lib/Drupal/Core/KeyValueStore/StateInterface.php
+++ b/core/lib/Drupal/Core/KeyValueStore/StateInterface.php
@@ -68,4 +68,11 @@ public function delete($key);
    */
   public function deleteMultiple(array $keys);
 
+  /**
+   * Resets the static cache.
+   *
+   * This is only needed for Simpletest.
+   */
+  public function resetCache();
+
 }
diff --git a/core/modules/block/lib/Drupal/block/Tests/BlockLanguageTest.php b/core/modules/block/lib/Drupal/block/Tests/BlockLanguageTest.php
index 915e4bb..fefb9fd 100644
--- a/core/modules/block/lib/Drupal/block/Tests/BlockLanguageTest.php
+++ b/core/modules/block/lib/Drupal/block/Tests/BlockLanguageTest.php
@@ -75,6 +75,7 @@ public function testLanguageBlockVisibility() {
 
     // Reset the static cache of the language list.
     drupal_static_reset('language_list');
+    \Drupal::state()->resetCache();
 
     // Check that a page has a block.
     $this->drupalget('', array('language' => language_load('en')));
diff --git a/core/modules/file/lib/Drupal/file/Tests/FileManagedTestBase.php b/core/modules/file/lib/Drupal/file/Tests/FileManagedTestBase.php
index 96305bd..b6fb820 100644
--- a/core/modules/file/lib/Drupal/file/Tests/FileManagedTestBase.php
+++ b/core/modules/file/lib/Drupal/file/Tests/FileManagedTestBase.php
@@ -38,6 +38,8 @@ function setUp() {
    *   'insert', etc.
    */
   function assertFileHooksCalled($expected) {
+    \Drupal::state()->resetCache();
+
     // Determine which hooks were called.
     $actual = array_keys(array_filter(file_test_get_all_calls()));
 
diff --git a/core/modules/language/lib/Drupal/language/Tests/LanguageConfigurationTest.php b/core/modules/language/lib/Drupal/language/Tests/LanguageConfigurationTest.php
index 1fb8c85..3381d44 100644
--- a/core/modules/language/lib/Drupal/language/Tests/LanguageConfigurationTest.php
+++ b/core/modules/language/lib/Drupal/language/Tests/LanguageConfigurationTest.php
@@ -72,6 +72,8 @@ function testLanguageConfiguration() {
     );
     $this->drupalPostForm(NULL, $edit, t('Save configuration'));
     $this->assertOptionSelected('edit-site-default-language', 'fr', 'Default language updated.');
+
+    \Drupal::state()->resetCache();
     $this->assertEqual($this->getUrl(), url('admin/config/regional/settings', array('absolute' => TRUE)), 'Correct page redirection.');
 
     // Check if a valid language prefix is added after changing the default
diff --git a/core/modules/language/lib/Drupal/language/Tests/LanguageListTest.php b/core/modules/language/lib/Drupal/language/Tests/LanguageListTest.php
index 8b2190d..b05808d 100644
--- a/core/modules/language/lib/Drupal/language/Tests/LanguageListTest.php
+++ b/core/modules/language/lib/Drupal/language/Tests/LanguageListTest.php
@@ -72,6 +72,7 @@ function testLanguageList() {
     );
     $this->drupalPostForm(NULL, $edit, t('Save configuration'));
     $this->assertNoOptionSelected('edit-site-default-language', 'en', 'Default language updated.');
+    \Drupal::state()->resetCache();
     $this->assertEqual($this->getUrl(), url($path, array('absolute' => TRUE)), 'Correct page redirection.');
 
     // Ensure we can't delete the default language.
@@ -96,6 +97,7 @@ function testLanguageList() {
       'site_default_language' => 'en',
     );
     $this->drupalPostForm($path, $edit, t('Save configuration'));
+    \Drupal::state()->resetCache();
     // Ensure 'delete' link works.
     $this->drupalGet('admin/config/regional/language');
     $this->clickLink(t('Delete'));
@@ -125,6 +127,7 @@ function testLanguageList() {
     $this->drupalPostForm('admin/config/regional/language/delete/fr', array(), t('Delete'));
     // Get the count of languages.
     drupal_static_reset('language_list');
+    \Drupal::state()->resetCache();
     $languages = language_list();
     // We need raw here because %language and %langcode will add HTML.
     $t_args = array('%language' => 'French', '%langcode' => 'fr');
diff --git a/core/modules/locale/lib/Drupal/locale/Tests/LocaleImportFunctionalTest.php b/core/modules/locale/lib/Drupal/locale/Tests/LocaleImportFunctionalTest.php
index 8cd217b..dff7ec3 100644
--- a/core/modules/locale/lib/Drupal/locale/Tests/LocaleImportFunctionalTest.php
+++ b/core/modules/locale/lib/Drupal/locale/Tests/LocaleImportFunctionalTest.php
@@ -136,6 +136,7 @@ function testStandalonePoFile() {
     $this->drupalPostForm('admin/config/regional/translate', $search, t('Filter'));
     $this->assertNoText(t('No strings available.'), 'String overwritten by imported string.');
     // This import should have changed number of plural forms.
+    \Drupal::state()->resetCache();
     $locale_plurals = \Drupal::state()->get('locale.translation.plurals') ?: array();
     $this->assert($locale_plurals['fr']['plurals'] == 3, 'Plural numbers changed.');
 
diff --git a/core/modules/locale/lib/Drupal/locale/Tests/LocaleUpdateTest.php b/core/modules/locale/lib/Drupal/locale/Tests/LocaleUpdateTest.php
index 3d20f6d..1ed7800 100644
--- a/core/modules/locale/lib/Drupal/locale/Tests/LocaleUpdateTest.php
+++ b/core/modules/locale/lib/Drupal/locale/Tests/LocaleUpdateTest.php
@@ -130,6 +130,7 @@ function testUpdateCheckStatus() {
 
     // Get status of translation sources at both local and remote locations.
     $this->drupalGet('admin/reports/translations/check');
+    \Drupal::state()->resetCache();
     $result = locale_translation_get_status();
     $this->assertEqual($result['contrib_module_one']['de']->type, LOCALE_TRANSLATION_REMOTE, 'Translation of contrib_module_one found');
     $this->assertEqual($result['contrib_module_one']['de']->timestamp, $this->timestamp_new, 'Translation timestamp found');
diff --git a/core/modules/node/lib/Drupal/node/Tests/NodeTypePersistenceTest.php b/core/modules/node/lib/Drupal/node/Tests/NodeTypePersistenceTest.php
index 66ea769..f07f3e1 100644
--- a/core/modules/node/lib/Drupal/node/Tests/NodeTypePersistenceTest.php
+++ b/core/modules/node/lib/Drupal/node/Tests/NodeTypePersistenceTest.php
@@ -55,6 +55,7 @@ function testNodeTypeCustomizationPersistence() {
     $this->drupalPostForm('admin/modules/uninstall', $edit, t('Uninstall'));
     $this->drupalPostForm(NULL, array(), t('Uninstall'));
     $forum = entity_load('node_type', 'forum');
+    \Drupal::state()->resetCache();
     $this->assertFalse($forum->isLocked(), 'Forum node type is not locked');
     $this->drupalGet('node/add');
     $this->assertNoText('forum', 'forum type is no longer found on node/add');
diff --git a/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php b/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
index 42f217d..20f000a 100644
--- a/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
@@ -2367,6 +2367,7 @@ protected function drupalGetSettings() {
    *   An array containing e-mail messages captured during the current test.
    */
   protected function drupalGetMails($filter = array()) {
+    \Drupal::state()->resetCache();
     $captured_emails = \Drupal::state()->get('system.test_email_collector') ?: array();
     $filtered_emails = array();
 
diff --git a/core/modules/system/lib/Drupal/system/Tests/Batch/ProcessingTest.php b/core/modules/system/lib/Drupal/system/Tests/Batch/ProcessingTest.php
index b452e69..be395541 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Batch/ProcessingTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Batch/ProcessingTest.php
@@ -54,6 +54,7 @@ function testBatchForm() {
     $edit = array('batch' => 'batch_1');
     $this->drupalPostForm('batch-test/simple', $edit, 'Submit');
     $this->assertBatchMessages($this->_resultMessages('batch_1'), 'Batch with simple operations performed successfully.');
+    \Drupal::state()->resetCache();
     $this->assertEqual(batch_test_stack(), $this->_resultStack('batch_1'), 'Execution order was correct.');
     $this->assertText('Redirection successful.', 'Redirection after batch execution is correct.');
 
@@ -61,6 +62,7 @@ function testBatchForm() {
     $edit = array('batch' => 'batch_2');
     $this->drupalPostForm('batch-test/simple', $edit, 'Submit');
     $this->assertBatchMessages($this->_resultMessages('batch_2'), 'Batch with multistep operation performed successfully.');
+    \Drupal::state()->resetCache();
     $this->assertEqual(batch_test_stack(), $this->_resultStack('batch_2'), 'Execution order was correct.');
     $this->assertText('Redirection successful.', 'Redirection after batch execution is correct.');
 
@@ -68,6 +70,7 @@ function testBatchForm() {
     $edit = array('batch' => 'batch_3');
     $this->drupalPostForm('batch-test/simple', $edit, 'Submit');
     $this->assertBatchMessages($this->_resultMessages('batch_3'), 'Batch with simple and multistep operations performed successfully.');
+    \Drupal::state()->resetCache();
     $this->assertEqual(batch_test_stack(), $this->_resultStack('batch_3'), 'Execution order was correct.');
     $this->assertText('Redirection successful.', 'Redirection after batch execution is correct.');
 
@@ -75,6 +78,7 @@ function testBatchForm() {
     $edit = array('batch' => 'batch_4');
     $this->drupalPostForm('batch-test/simple', $edit, 'Submit');
     $this->assertBatchMessages($this->_resultMessages('batch_4'), 'Nested batch performed successfully.');
+    \Drupal::state()->resetCache();
     $this->assertEqual(batch_test_stack(), $this->_resultStack('batch_4'), 'Execution order was correct.');
     $this->assertText('Redirection successful.', 'Redirection after batch execution is correct.');
   }
diff --git a/core/modules/system/lib/Drupal/system/Tests/Entity/EntityTranslationFormTest.php b/core/modules/system/lib/Drupal/system/Tests/Entity/EntityTranslationFormTest.php
index 68f6303..9407d70 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Entity/EntityTranslationFormTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Entity/EntityTranslationFormTest.php
@@ -113,6 +113,7 @@ function testEntityFormLanguage() {
     $node->getTranslation($langcode2)->body->value = $this->randomName(16);
     $node->save();
     $this->drupalGet($langcode2 . '/node/' . $node->id() . '/edit');
+    \Drupal::state()->resetCache();
     $form_langcode = \Drupal::state()->get('entity_test.form_langcode') ?: FALSE;
     $this->assertTrue($langcode2 == $form_langcode, "Node edit form language is $langcode2.");
   }
diff --git a/core/modules/system/lib/Drupal/system/Tests/Menu/TrailTest.php b/core/modules/system/lib/Drupal/system/Tests/Menu/TrailTest.php
index 111c118..374077c 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Menu/TrailTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Menu/TrailTest.php
@@ -156,6 +156,7 @@ function testCustom403And404Pages() {
 
       // Check that the initial trail (during the Drupal bootstrap) matches
       // what we expect.
+      \Drupal::state()->resetCache();
       $initial_trail = \Drupal::state()->get('menu_test.active_trail_initial') ?: array();
       $this->assertEqual(count($initial_trail), count($expected_trail[$status_code]['initial']), format_string('The initial active trail for a @status_code page contains the expected number of items (expected: @expected, found: @found).', array(
         '@status_code' => $status_code,
diff --git a/core/modules/system/lib/Drupal/system/Tests/System/CronRunTest.php b/core/modules/system/lib/Drupal/system/Tests/System/CronRunTest.php
index 6fe28f2..996dea4 100644
--- a/core/modules/system/lib/Drupal/system/Tests/System/CronRunTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/System/CronRunTest.php
@@ -68,6 +68,7 @@ function testAutomaticCron() {
     \Drupal::state()->set('system.cron_last', $cron_last);
     $this->drupalGet('');
     sleep(1);
+    \Drupal::state()->resetCache();
     $this->assertTrue($cron_last < \Drupal::state()->get('system.cron_last'), 'Cron runs when the cron threshold is passed.');
 
     // Disable the cron threshold through the interface.
diff --git a/core/tests/Drupal/Tests/Core/PrivateKeyTest.php b/core/tests/Drupal/Tests/Core/PrivateKeyTest.php
index a77a939..8e62f1c 100644
--- a/core/tests/Drupal/Tests/Core/PrivateKeyTest.php
+++ b/core/tests/Drupal/Tests/Core/PrivateKeyTest.php
@@ -52,7 +52,7 @@ public function setUp() {
     parent::setUp();
     $this->key = Crypt::randomStringHashed(55);
 
-    $this->state = $this->getMock('Drupal\Core\KeyValueStore\KeyValueStoreInterface');
+    $this->state = $this->getMock('Drupal\Core\KeyValueStore\StateInterface');
 
     $this->privateKey = new PrivateKey($this->state);
   }