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

vendor/stack/builder/src/Stack/Builder.php

@@ -0,0 +1,63 @@
+<?php
+
+namespace Stack;
+
+use Symfony\Component\HttpKernel\HttpKernelInterface;
+
+class Builder
+{
+    private $specs;
+
+    public function __construct()
+    {
+        $this->specs = new \SplStack();
+    }
+
+    public function unshift(/*$kernelClass, $args...*/)
+    {
+        if (func_num_args() === 0) {
+            throw new \InvalidArgumentException("Missing argument(s) when calling unshift");
+        }
+
+        $spec = func_get_args();
+        $this->specs->unshift($spec);
+
+        return $this;
+    }
+
+    public function push(/*$kernelClass, $args...*/)
+    {
+        if (func_num_args() === 0) {
+            throw new \InvalidArgumentException("Missing argument(s) when calling push");
+        }
+
+        $spec = func_get_args();
+        $this->specs->push($spec);
+
+        return $this;
+    }
+
+    public function resolve(HttpKernelInterface $app)
+    {
+        $middlewares = array($app);
+
+        foreach ($this->specs as $spec) {
+            $args = $spec;
+            $firstArg = array_shift($args);
+
+            if (is_callable($firstArg)) {
+                $app = $firstArg($app);
+            } else {
+                $kernelClass = $firstArg;
+                array_unshift($args, $app);
+
+                $reflection = new \ReflectionClass($kernelClass);
+                $app = $reflection->newInstanceArgs($args);
+            }
+
+            array_unshift($middlewares, $app);
+        }
+
+        return new StackedHttpKernel($app, $middlewares);
+    }
+}