Update to drupal 8.0.0-rc1. For more information, see https://www.drupal.org/node/2582663

vendor/psr/log/Psr/Log/LogLevel.php

@@ -0,0 +1,18 @@
+<?php
+
+namespace Psr\Log;
+
+/**
+ * Describes log levels
+ */
+class LogLevel
+{
+    const EMERGENCY = 'emergency';
+    const ALERT = 'alert';
+    const CRITICAL = 'critical';
+    const ERROR = 'error';
+    const WARNING = 'warning';
+    const NOTICE = 'notice';
+    const INFO = 'info';
+    const DEBUG = 'debug';
+}