diff -r e77dad242f4c -r 2a9ec03fe5a1 vendor/github.com/gorilla/websocket/mask_safe.go
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/vendor/github.com/gorilla/websocket/mask_safe.go	Sat Sep 29 18:09:54 2018 +0200
@@ -0,0 +1,15 @@
+// Copyright 2016 The Gorilla WebSocket Authors. All rights reserved.  Use of
+// this source code is governed by a BSD-style license that can be found in the
+// LICENSE file.
+
+// +build appengine
+
+package websocket
+
+func maskBytes(key [4]byte, pos int, b []byte) int {
+	for i := range b {
+		b[i] ^= key[pos&3]
+		pos++
+	}
+	return pos & 3
+}