PolyGerrit: Use es5 syntax instead of es6

es6 is currently breaking in browsers so lets use es5 for now.

Change-Id: I918c79ceb24490e85870a8b30bf4506acb54c391

polygerrit-ui/app/elements/plugins/gr-external-style/gr-external-style.js

@@ -22,9 +22,9 @@
     },
 
     _import: function(url) {
-      return new Promise((resolve, reject) => {
+      return new Promise(function(resolve, reject) {
         this.importHref(url, resolve, reject);
-      });
+      }.bind(this));
     },
 
     _applyStyle: function(name) {