Load babel-polyfill only when necessary.

public/assets/polyfill.js

@@ -0,0 +1,10 @@
+// stupid old browsers
+if(typeof Promise === 'undefined') {
+  (function(d, script) {
+    script = d.createElement('script');
+    script.type = 'text/javascript';
+    script.async = false;
+    script.src = '/assets/babel-polyfill.js';
+    d.getElementsByTagName('head')[0].appendChild(script);
+  }(document));
+}