diff --git a/core/lib/Drupal/Core/Session/AccountProxy.php b/core/lib/Drupal/Core/Session/AccountProxy.php
index 2c00e62..dbe165e 100644
--- a/core/lib/Drupal/Core/Session/AccountProxy.php
+++ b/core/lib/Drupal/Core/Session/AccountProxy.php
@@ -108,11 +108,11 @@ public function getAccount() {
    */
   public function impersonateAccount(AccountInterface $account) {
     // Prevent session information from being saved and push the previous account.
-    $this->accountStack[] = $this->account;
+    array_push($this->accountStack, $this->account);
     $this->originalSessionSaving = $this->sessionManager->isEnabled();
     $this->sessionManager->disable();
     $this->setAccount($account);
-    return $this->account;
+    return $this;
   }
 
   /**
@@ -123,30 +123,35 @@ public function revertAccount() {
     if (!empty($this->accountStack)) {
       $this->account = array_pop($this->accountStack);
     }
+    else {
+      throw new \RuntimeException(t('No more account impersonations to revert.'));
+    }
     // Restore original session saving status if all impersonations are reverted.
     if (empty($this->accountStack)) {
       if ($this->originalSessionSaving) {
         $this->sessionManager->enable();
       }
     }
-    return $this->account;
+    return $this;
   }
 
   /**
-  /**
    * {@inheritdoc}
    */
   public function revertAll() {
     // Restore the original account from the stack.
     if (!empty($this->accountStack)) {
-      $this->account = $this->accountStack[0];
+      $this->account = array_shift($this->accountStack);
       $this->accountStack = array();
     }
+    else {
+      throw new \RuntimeException(t('No more account impersonations to revert.'));
+    }
     // Restore original session saving status if all impersonations are reverted.
     if ($this->originalSessionSaving) {
       $this->sessionManager->enable();
     }
-    return $this->account;
+    return $this;
   }
 
   /**
diff --git a/core/lib/Drupal/Core/Session/AccountProxyInterface.php b/core/lib/Drupal/Core/Session/AccountProxyInterface.php
index 9558d65..c4cf390 100644
--- a/core/lib/Drupal/Core/Session/AccountProxyInterface.php
+++ b/core/lib/Drupal/Core/Session/AccountProxyInterface.php
@@ -38,7 +38,7 @@ public function getAccount();
    * call!
    *
    * @param \Drupal\Core\Session\AccountInterface
-   *   The account to be impersonated.
+   *   $this.
    */
   public function impersonateAccount(AccountInterface $account);
 
@@ -46,7 +46,7 @@ public function impersonateAccount(AccountInterface $account);
    * Revert from impersonating another account.
    *
    * @return \Drupal\Core\Session\AccountInterface
-   *   The last account that was impersonated.
+   *   $this.
    */
   public function revertAccount();
 
@@ -57,7 +57,7 @@ public function revertAccount();
    * been possibility of multiple impersonations.
    *
    * @return \Drupal\Core\Session\AccountInterface
-   *   The original account that was impersonated.
+   *   $this.
    */
   public function revertAll();
 }
diff --git a/core/modules/simpletest/src/TestBase.php b/core/modules/simpletest/src/TestBase.php
index b4d26a5..50f28a2 100644
--- a/core/modules/simpletest/src/TestBase.php
+++ b/core/modules/simpletest/src/TestBase.php
@@ -1082,9 +1082,18 @@ private function prepareEnvironment() {
     $request = Request::create('/');
     $this->container->set('request', $request);
 
+    // Copy the current session and authentication managers into the new environment.
+    $this->container->set('session_manager', \Drupal::service('session_manager'));
+    $this->container->set('authentication', \Drupal::service('authentication'));
+
+    // Create new current user service for the test environment.
+    $this->container->register('current_user', '\Drupal\Core\Session\AccountProxy')
+      ->addArgument(new Reference('authentication'))
+      ->addArgument(new Reference('request'))
+      ->addArgument(new Reference('session_manager'));
+
     // Run all tests as an anonymous user by default, web tests will replace that
     // during the test set up.
-    $this->container->set('current_user', \Drupal::currentUser());
     $this->container->get('current_user')->impersonateAccount(new AnonymousUserSession());
 
     \Drupal::setContainer($this->container);
@@ -1221,9 +1230,6 @@ private function restoreEnvironment() {
     // Restore original shutdown callbacks.
     $callbacks = &drupal_register_shutdown_function();
     $callbacks = $this->originalShutdownCallbacks;
-
-    // Restore original user session.
-    \Drupal::currentUser()->revertAll();
   }
 
   /**
diff --git a/core/modules/system/lib/Drupal/system/Tests/Session/UserImpersonatingUserTest.php b/core/modules/system/lib/Drupal/system/Tests/Session/UserImpersonatingUserTest.php
index 1aa319e..2e08b23 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Session/UserImpersonatingUserTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Session/UserImpersonatingUserTest.php
@@ -5,7 +5,7 @@
  * Contains Drupal\user\Tests\UserImpersonatingUserTest.
  */
 
-namespace Drupal\user\Tests;
+namespace Drupal\system\Tests\Session;
 
 use Drupal\Core\Session\AnonymousUserSession;
 use Drupal\Core\Session\UserSession;
@@ -19,8 +19,8 @@ class UserImpersonatingUserTest extends WebTestBase {
   public static function getInfo() {
     return array(
       'name'        => 'Impersonate users',
-      'description' => 'Temporarily impersonate another user, and then restore the original user.',
-      'group'       => 'User',
+      'description' => 'Temporarily impersonate another user account, and then restore the original account.',
+      'group'       => 'Session',
     );
   }
 
@@ -29,10 +29,13 @@ function setUp() {
   }
 
   function testUserImpersonateUser() {
-    $user = \Drupal::currentUser();
+    $session_manager = $this->container->get('session_manager');
+    $user = $this->container->get('current_user');
     $original_user = clone $user;
+    $this->verbose('Session saving status: '. var_dump($session_manager->isEnabled()));
+    $this->assertFalse($session_manager->isEnabled(), 'Session saving is initially disabled.');
 
-    // If not currently logged in, use user_user_impersonate_user() to switch to
+    // If not currently logged in, use AccountProxy::impersonateAccount() to switch to
     // user 1. If logged in, switch to the anonymous user instead.
     if ($user->isAnonymous()) {
       $user->impersonateAccount(new UserSession(array('uid' => 1)));
@@ -42,12 +45,15 @@ function testUserImpersonateUser() {
 
     // Verify that the active user has changed, and that session saving is
     // disabled.
-    $this->assertEqual($user->id(), ($original_user->id() == 0 ? 1 : 0), t('User switched'));
-    $this->assertFalse(\Drupal::service('session_manager')->isEnabled(), t('Session saving is disabled.'));
+    $this->assertEqual($user->id(), ($original_user->id() == 0 ? 1 : 0), 'User switched');
+    $this->assertFalse($session_manager->isEnabled(), 'Session saving is disabled.');
+
+    // Enable session saving for the purpose of this test.
+    $session_manager->enable();
 
     // Perform a second (nested) impersonation.
     $user->impersonateAccount(new UserSession(array('uid' => 2)));
-    $this->assertEqual($user->id(), 2, t('User switched.'));
+    $this->assertEqual($user->id(), 2, 'User switched.');
 
     // Revert to the user which was active between the first and second
     // impersonation attempt.
@@ -55,8 +61,8 @@ function testUserImpersonateUser() {
 
     // Since we are still impersonating the user from the first attempt,
     // session handling still needs to be disabled.
-    $this->assertEqual($user->id(), ($original_user->id() == 0 ? 1 : 0), t('User switched.'));
-    $this->assertFalse(\Drupal::service('session_manager')->isEnabled(), t('Session saving is disabled.'));
+    $this->assertEqual($user->id(), ($original_user->id() == 0 ? 1 : 0), 'User switched.');
+    $this->assertFalse($session_manager->isEnabled(), 'Session saving is disabled.');
 
     // Revert to the original user which was active before the first
     // impersonation attempt.
@@ -64,12 +70,26 @@ function testUserImpersonateUser() {
 
     // Assert that the original user is the active user again, and that session
     // saving has been re-enabled.
-    $this->assertEqual($user->id(), $original_user->id(), t('Original user successfully restored.'));
+    $this->assertEqual($user->id(), $original_user->id(), 'Original user successfully restored.');
+    $this->assertTrue($session_manager->isEnabled(), 'Session saving is enabled.');
 
-    // Simpletest uses user_impersonate_user() too, revert the impersonation by
-    // Simpletest to enable session saving again. This is safe because calling
-    // user_revert_user() too often simply results in returning the active user.
-    $user->revertAccount();
-    $this->assertTrue(\Drupal::service('session_manager')->isEnabled(), t('Session saving is enabled.'));
+    // Verify that AccountProxy::revertAccount and AccountProxy::revertAll()
+    // will throw exceptions if there is none left in the stack.
+    try {
+      $ex1 = new \RuntimeException();
+      $user->revertAccount();
+    }
+    catch (\RuntimeException $e) {
+      $ex1 = $e;
+    }
+    $this->assertEqual($ex1->getMessage(), 'No more account impersonations to revert.', 'Revert account throws exception if called without previous impersonation.');
+    try {
+      $ex2 = new \RuntimeException();
+      $user->revertAll();
+    }
+    catch (\RuntimeException $e) {
+      $ex2 = $e;
+    }
+    $this->assertEqual($ex2->getMessage(), 'No more account impersonations to revert.', 'Revert all throws exception if called without previous impersonation.');
   }
 }