Only polyfill Promise if it doesn't exist at all

Author: shockey

src/polyfills.js

@@ -1,2 +1,5 @@
 // Promise global, Used ( at least ) by 'whatwg-fetch'. And required by IE 11
-require("core-js/fn/promise")
+
+if(!window.Promise) {
+  require("core-js/fn/promise")
+}