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.

src/polyfills.js

@@ -0,0 +1,2 @@
+// Promise global, Used ( at least ) by 'whatwg-fetch'. And required by IE 11
+require("core-js/fn/promise")