title: The Mix manifest does not exist when it does exist
body: <p>For my admin panel I extract all the assets including the <code>manifest-json.js</code> to <code>mix.setPublicPath(path.normalize('public/backend/'))</code>.</p>
  
  <p>All the files get correctly added to the backend folder, and the <code>manifest-json.js</code> file looks as follows:</p>
  
  <pre><code>{
      // all correct here
      "/js/vendor.js": "/js/vendor.js",
      "/js/app.js": "/js/app.js",
      "/css/app.css": "/css/app.css",
      "/js/manifest.js": "/js/manifest.js"
  }
  </code></pre>
  
  <p>the problem is that when using </p>
  
  <pre><code>{{ mix('backend/css/app.css') }}
  </code></pre>
  
  <p>in my blade-files, it looks in <code>public/manifest-json.js</code> instead of looking for it in <code>backend/manifest-json.js</code>.</p>
  
  <p>How can I make sure the right <code>manifest-json.js</code> file is used?</p>