diff --git a/core/lib/Drupal/Component/PhpStorage/FileReadOnlyStorage.php b/core/lib/Drupal/Component/PhpStorage/FileReadOnlyStorage.php
index 1a460ad..0325f4f 100644
--- a/core/lib/Drupal/Component/PhpStorage/FileReadOnlyStorage.php
+++ b/core/lib/Drupal/Component/PhpStorage/FileReadOnlyStorage.php
@@ -34,13 +34,6 @@ public function __construct(array $configuration) {
   }
 
   /**
-   * Implements Drupal\Component\PhpStorage\PhpStorageInterface::exists().
-   */
-  public function exists($name) {
-    return file_exists($this->getFullPath($name));
-  }
-
-  /**
    * Implements Drupal\Component\PhpStorage\PhpStorageInterface::load().
    */
   public function load($name) {
@@ -50,6 +43,13 @@ public function load($name) {
   }
 
   /**
+   * {@inheritdoc}
+   */
+  public function loadClass($name, $class_name, $wait = 200000, $retry = 5) {
+    return class_exists($class_name, FALSE) || $this->load($name);
+  }
+
+  /**
    * Implements Drupal\Component\PhpStorage\PhpStorageInterface::save().
    */
   public function save($name, $code) {
diff --git a/core/lib/Drupal/Component/PhpStorage/FileStorage.php b/core/lib/Drupal/Component/PhpStorage/FileStorage.php
index 71ff3f2..5cf604a 100644
--- a/core/lib/Drupal/Component/PhpStorage/FileStorage.php
+++ b/core/lib/Drupal/Component/PhpStorage/FileStorage.php
@@ -33,14 +33,30 @@ public function __construct(array $configuration) {
   }
 
   /**
-   * Implements Drupal\Component\PhpStorage\PhpStorageInterface::exists().
+   * {@inheritdoc}
    */
-  public function exists($name) {
-    return file_exists($this->getFullPath($name));
+  public function loadClass($name, $class_name = NULL, $wait = 200000, $retry = 5) {
+    while (!($exists = class_exists($class_name, FALSE)) && $this->doLoadClass($name, TRUE) && !($exists = class_exists($class_name, FALSE)) && $retry--) {
+      usleep(mt_rand($wait, 2 * $wait));
+    }
+    return $exists;
   }
 
   /**
-   * Implements Drupal\Component\PhpStorage\PhpStorageInterface::load().
+   * Do the actual class loading.
+   *
+   * @param $name
+   *   The name of the file.
+   *
+   * @return bool
+   *   TRUE on success, FALSE otherwise.
+   */
+  protected function doLoadClass($name) {
+    return (@include $this->getFullPath($name)) !== FALSE;
+  }
+
+  /**
+   * {@inheritdoc}
    */
   public function load($name) {
     // The FALSE returned on failure is enough for the caller to handle this,
diff --git a/core/lib/Drupal/Component/PhpStorage/MTimeProtectedFileStorage.php b/core/lib/Drupal/Component/PhpStorage/MTimeProtectedFileStorage.php
index 573e23c..da593b8 100644
--- a/core/lib/Drupal/Component/PhpStorage/MTimeProtectedFileStorage.php
+++ b/core/lib/Drupal/Component/PhpStorage/MTimeProtectedFileStorage.php
@@ -47,10 +47,14 @@ public function load($name) {
   }
 
   /**
-   * Implements Drupal\Component\PhpStorage\PhpStorageInterface::exists().
+   * {@inheritdoc}
    */
-  public function exists($name) {
-    return $this->checkFile($name) !== FALSE;
+  protected function doLoadClass($name) {
+    if (($filename = $this->checkFile($name)) !== FALSE) {
+      // Inline parent::load() to avoid an expensive getFullPath() call.
+      return (@include $filename) !== FALSE;
+    }
+    return FALSE;
   }
 
   /**
diff --git a/core/lib/Drupal/Component/PhpStorage/PhpStorageInterface.php b/core/lib/Drupal/Component/PhpStorage/PhpStorageInterface.php
index 1f6a682..a2da2cd 100644
--- a/core/lib/Drupal/Component/PhpStorage/PhpStorageInterface.php
+++ b/core/lib/Drupal/Component/PhpStorage/PhpStorageInterface.php
@@ -19,31 +19,48 @@
 interface PhpStorageInterface {
 
   /**
-   * Checks whether the PHP code exists in storage.
-   *
-   * @param string $name
-   *   The virtual file name. Can be a relative path.
-   *
-   * @return bool
-   *   TRUE if the virtual file exists, FALSE otherwise.
-   */
-  public function exists($name);
-
-  /**
    * Loads PHP code from storage.
    *
    * Depending on storage implementation, exists() checks can be expensive, so
    * this function may be called for a file that doesn't exist, and that should
-   * not result in errors. This function does not return anything, so it is
-   * up to the caller to determine if any code was loaded (for example, check
-   * class_exists() or function_exists() for what was expected in the code).
+   * not result in errors.
    *
    * @param string $name
    *   The virtual file name. Can be a relative path.
+   *
+   * @return
+   *   TRUE on success, FALSE otherwise. This only means a file was loaded, the
+   *   contents were not checked, that's up to the caller. Check the
+   *   loadClass() method for loading classes.
    */
   public function load($name);
 
   /**
+   * Loads a class from storage.
+   *
+   * Depending on storage implementation, exists() checks can be expensive, so
+   * this function may be called for a file that doesn't exist, and that should
+   * not result in errors.
+   *
+   * @param string $name
+   *   The virtual file name. Can be a relative path.
+   * @param string $class_name
+   *   A class name to verify after loading. In a race condition, the file
+   *   might exist but not contain the right value.
+   * @param int $wait
+   *   If the file is found but the class does not exist then a race condition
+     * is very likely so wait a random amount of time between $wait and
+     * 2 * $wait microseconds before returning to allow the racing process to
+     * finish.
+   * @param int $retry
+   *   The number of times to retry waiting and reloading.
+   *
+   * @return
+   *   TRUE on success, FALSE otherwise.
+   */
+  public function loadClass($name, $class_name, $wait = 200000, $retry = 5);
+
+  /**
    * Saves PHP code to storage.
    *
    * @param string $name
diff --git a/core/lib/Drupal/Core/DrupalKernel.php b/core/lib/Drupal/Core/DrupalKernel.php
index 557dfd1..6fcecbb 100644
--- a/core/lib/Drupal/Core/DrupalKernel.php
+++ b/core/lib/Drupal/Core/DrupalKernel.php
@@ -720,11 +720,12 @@ protected function initializeContainer($rebuild = FALSE) {
     // If the module list hasn't already been set in updateModules and we are
     // not forcing a rebuild, then try and load the container from the disk.
     if (empty($this->moduleList) && !$rebuild) {
-      $fully_qualified_class_name = '\\' . $this->getClassNamespace() . '\\' . $this->getClassName();
+      $class_name = $this->getClassName();
+      $fully_qualified_class_name = '\\' . $this->getClassNamespace() . '\\' . $class_name;
 
       // First, try to load from storage.
       if (!class_exists($fully_qualified_class_name, FALSE)) {
-        $this->storage()->load($this->getClassName() . '.php');
+        $this->storage()->load($class_name . '.php', $fully_qualified_class_name);
       }
       // If the load succeeded or the class already existed, use it.
       if (class_exists($fully_qualified_class_name, FALSE)) {
diff --git a/core/lib/Drupal/Core/Template/TwigEnvironment.php b/core/lib/Drupal/Core/Template/TwigEnvironment.php
index 9225712..3807888 100644
--- a/core/lib/Drupal/Core/Template/TwigEnvironment.php
+++ b/core/lib/Drupal/Core/Template/TwigEnvironment.php
@@ -127,9 +127,9 @@ public function loadTemplate($name, $index = NULL) {
           $this->updateCompiledTemplate($cache_filename, $name);
         }
 
-        if (!$this->storage()->load($cache_filename)) {
+        if (!$this->storage()->loadClass($cache_filename, $cls)) {
           $this->updateCompiledTemplate($cache_filename, $name);
-          $this->storage()->load($cache_filename);
+          $this->storage()->loadClass($cache_filename, $cls);
         }
       }
     }
diff --git a/core/modules/system/src/Tests/Theme/TwigSettingsTest.php b/core/modules/system/src/Tests/Theme/TwigSettingsTest.php
index 36cc48d..0ab9773 100644
--- a/core/modules/system/src/Tests/Theme/TwigSettingsTest.php
+++ b/core/modules/system/src/Tests/Theme/TwigSettingsTest.php
@@ -105,7 +105,7 @@ function testTwigCacheOverride() {
 
     // Navigate to the page and make sure the template gets cached.
     $this->drupalGet('theme-test/template-test');
-    $this->assertTrue(PhpStorageFactory::get('twig')->exists($cache_filename), 'Cached Twig template found.');
+    $this->assertTrue(PhpStorageFactory::get('twig')->load($cache_filename), 'Cached Twig template found.');
 
     // Disable the Twig cache and rebuild the service container.
     $parameters = $this->container->getParameter('twig.config');
diff --git a/core/tests/Drupal/Tests/Component/PhpStorage/FileStorageReadOnlyTest.php b/core/tests/Drupal/Tests/Component/PhpStorage/FileStorageReadOnlyTest.php
index 8cf9642..2516465 100644
--- a/core/tests/Drupal/Tests/Component/PhpStorage/FileStorageReadOnlyTest.php
+++ b/core/tests/Drupal/Tests/Component/PhpStorage/FileStorageReadOnlyTest.php
@@ -71,7 +71,7 @@ public function testReadOnly() {
 
     // If the file was successfully loaded, it must also exist, but ensure the
     // exists() method returns that correctly.
-    $this->assertSame($php_read->exists($name), TRUE);
+    $this->assertSame($php_read->load($name), TRUE);
     // Saving and deleting should always fail.
     $this->assertFalse($php_read->save($name, $code));
     $this->assertFalse($php_read->delete($name));