fix(menu): IE11 support

fix #673

src/resources/js/libs/custom-elements-es5-adapter.js

@@ -0,0 +1,15 @@
+/**
+@license @nocompile
+Copyright (c) 2018 The Polymer Project Authors. All rights reserved.
+This code may only be used under the BSD style license found at http://polymer.github.io/LICENSE.txt
+The complete set of authors may be found at http://polymer.github.io/AUTHORS.txt
+The complete set of contributors may be found at http://polymer.github.io/CONTRIBUTORS.txt
+Code distributed by Google as part of the polymer project is also
+subject to an additional IP rights grant found at http://polymer.github.io/PATENTS.txt
+*/
+(function () {
+  'use strict';
+
+  (function(){if(void 0===window.Reflect||void 0===window.customElements||window.customElements.hasOwnProperty('polyfillWrapFlushCallback'))return;const a=HTMLElement;window.HTMLElement=function(){return Reflect.construct(a,[],this.constructor)},HTMLElement.prototype=a.prototype,HTMLElement.prototype.constructor=HTMLElement,Object.setPrototypeOf(HTMLElement,a);})();
+
+}());

src/templates/page.hbs

@@ -155,7 +155,9 @@
 
        <script src="{{relativeURL data.depth }}js/libs/custom-elements.min.js"></script>
        <script src="{{relativeURL data.depth }}js/libs/lit-html.js"></script>
-       <script src="{{relativeURL data.depth }}js/menu-wc.js"></script>
+       <!-- Required to polyfill modern browsers as code is ES5 for IE... -->
+       <script src="{{relativeURL data.depth }}js/libs/custom-elements-es5-adapter.js" charset="utf-8" defer></script>
+       <script src="{{relativeURL data.depth }}js/menu-wc.js" defer></script>
 
        <script src="{{relativeURL data.depth }}js/libs/bootstrap-native.js"></script>