Fix startup race condition

Summary:
Currently Flipper.init is called as soon as the script loads.
However, if the store takes a bit longer than usual to get rehydrated, then this init call can get called before it has been set.

Fixing it by using a specific event instead of the generic `onload`.

Reviewed By: passy

Differential Revision: D17711399

fbshipit-source-id: fcaf9e5943bfd15359038b8e3722f11d7e06379b

src/init.tsx

@@ -88,4 +88,5 @@ persistStore(store, undefined, () => {
   dispatcher(store, logger);
   // make init function callable from outside
   window.Flipper.init = init;
+  window.dispatchEvent(new Event('flipper-store-ready'));
 });