diff -u b/core/lib/Drupal/Component/PhpLoader/MTimeProtectedLoader.php b/core/lib/Drupal/Component/PhpLoader/MTimeProtectedLoader.php
--- b/core/lib/Drupal/Component/PhpLoader/MTimeProtectedLoader.php
+++ b/core/lib/Drupal/Component/PhpLoader/MTimeProtectedLoader.php
@@ -55,6 +55,7 @@
       include_once $filename;
       return TRUE;
     }
+    return FALSE;
   }
 
   /**
@@ -67,7 +68,7 @@
       mkdir($this->prefix, 0700, TRUE);
       chmod($this->prefix, 0700);
     }
-    if (!$length = @file_put_contents($original_path, $data)) {
+    if (!@file_put_contents($original_path, $data)) {
       return FALSE;
     }
     chmod($original_path, 0400);
@@ -109,7 +110,7 @@
       chmod($dir, 0100);
       $loop++;
     }
-    return $length;
+    return TRUE;
   }
 
   /**
@@ -118,7 +119,7 @@
   public function delete($filename) {
     $dir = $this->prefix . '/' . str_replace('/', '#', $filename);
     $this->cleanDir($dir);
-    rmdir($dir);
+    return rmdir($dir);
   }
 
   /**
diff -u b/core/lib/Drupal/Component/PhpLoader/NativeLoader.php b/core/lib/Drupal/Component/PhpLoader/NativeLoader.php
--- b/core/lib/Drupal/Component/PhpLoader/NativeLoader.php
+++ b/core/lib/Drupal/Component/PhpLoader/NativeLoader.php
@@ -1,4 +1,5 @@
 <?php
+
 /**
  * @file
  * Definition of Drupal\Component\PhpLoader\NativeLoader.
@@ -32,8 +33,12 @@
    * Implements Drupal\Component\PhpLoader\PhpLoaderInterface::include()
    */
   public function includePhp($filename) {
-    include_once $this->prefix . '/' . $filename;
-    return TRUE;
+    $path = $this->prefix . '/' . $filename;
+    if (file_exists($path)) {
+      include_once $path;
+      return TRUE;
+    }
+    return FALSE;
   }
 
   /**
@@ -43,12 +48,12 @@
     $path = $this->prefix . '/' . $filename;
     @mkdir(dirname($path), 0700, TRUE);
-    return file_put_contents($path, $data);
+    return (bool) file_put_contents($path, $data);
   }
 
   /**
    * Implements Drupal\Component\PhpLoader\PhpLoaderInterface::delete()
    */
   public function delete($filename) {
-    unlink($filename);
+    return unlink($filename);
   }
 }
diff -u b/core/lib/Drupal/Component/PhpLoader/PhpLoaderInterface.php b/core/lib/Drupal/Component/PhpLoader/PhpLoaderInterface.php
--- b/core/lib/Drupal/Component/PhpLoader/PhpLoaderInterface.php
+++ b/core/lib/Drupal/Component/PhpLoader/PhpLoaderInterface.php
@@ -18,8 +18,8 @@
    * @param string $filename
    *   The filename. Can be a relative path.
    *
-   * @return
-   *   TRUE if the include was successful.
+   * @return bool
+   *   TRUE if the include succeeded, FALSE if it failed.
    */
   public function includePhp($filename);
 
@@ -31,9 +31,8 @@
    * @param string $data
    *    The PHP code to be written.
    *
-   * @return
-   *   FALSE if the write failed, anything but FALSE on success. For example,
-   *   core implementations return the number of bytes written.
+   * @return bool
+   *   TRUE if the write succeeded, FALSE if it failed.
    */
   public function write($filename, $data);
 
@@ -44,6 +43,9 @@
    *   The filename to be deleted. Can be a relative path.
    * @param string $bin
    *   An optional bin. Separate bins can use a different loader class.
+   *
+   * @return bool
+   *   TRUE if the delete succeeded, FALSE if it failed.
    */
   public function delete($filename);
 }