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

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

@@ -0,0 +1,27 @@
+<?php
+
+namespace Psr\Log;
+
+/**
+ * This Logger can be used to avoid conditional log calls
+ *
+ * Logging should always be optional, and if no logger is provided to your
+ * library creating a NullLogger instance to have something to throw logs at
+ * is a good way to avoid littering your code with `if ($this->logger) { }`
+ * blocks.
+ */
+class NullLogger extends AbstractLogger
+{
+    /**
+     * Logs with an arbitrary level.
+     *
+     * @param mixed $level
+     * @param string $message
+     * @param array $context
+     * @return null
+     */
+    public function log($level, $message, array $context = array())
+    {
+        // noop
+    }
+}