Add polyfill for Promise ( only )

This is needed for whatwg-fetch + IE11.
An alternative is to include "node_modules/whatwg-fetch" in the
transform-runtime.
But my guess is that someone is likely going to add a lib that in turn
uses Promises, without adding it to the whitelist. This is safter.

webpack-hot-dev-server.config.js

@@ -10,10 +10,12 @@ module.exports = require("./make-webpack-config")({
 	devtool: "eval",
   entry: {
     'swagger-ui-bundle': [
+      './src/polyfills',
       './src/core/index.js'
     ],
     'swagger-ui-standalone-preset': [
       'webpack/hot/dev-server',
+      './src/polyfills',
       './src/standalone/index.js',
     ]
   },