Enable downlevelIteration

Summary:
Brings us closer to what we had with Flow where
we can directly iterate over maps etc.

More detail here: https://mariusschulz.com/blog/downlevel-iteration-for-es3-es5-in-typescript

Reviewed By: danielbuechele

Differential Revision: D16647222

fbshipit-source-id: 22732c8667e13b43a0f2f9cf7ad036369575cc5b

tsconfig.json

@@ -10,6 +10,7 @@
     "jsx": "react",
     "moduleResolution": "node",
     "baseUrl": ".",
+    "downlevelIteration": true,
     "paths": {
       "flipper": ["./src/index.js"]
     }