message: Add missing <custom-style> to polymer-2-element demo template
diff: <link rel="import" href="../../iron-demo-helpers/demo-snippet.html">
  <link rel="import" href="../<%= name %>.html">
  + <custom-style>
  <style is="custom-style" include="demo-pages-shared-styles">
  </style>
  + </custom-style>
  </head>
  <body>
  <div class="vertical-section-container centered">
  