Make examples work in Internet Explorer

examples/mapbox-style.html

@@ -18,6 +18,7 @@ cloak:
     <meta name="viewport" content="initial-scale=1.0, user-scalable=no, width=device-width">
     <title>Mapbox Style objects with ol-mapbox-style</title>
     <link rel="stylesheet" href="../css/ol.css" type="text/css">
+    <script src="https://cdn.polyfill.io/v2/polyfill.min.js?features=fetch,String.prototype.startsWith,Object.assign"></script>
     <style type="text/css">
       html, body, .map {
         margin: 0;