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

static/index.dev.html

@@ -85,13 +85,10 @@
             openError('Script failure. Check Chrome console for more info.');
           };
 
-          script.onload = () => {
-            global.Flipper.init();
-          };
+          window.addEventListener('flipper-store-ready', () => global.Flipper.init());
 
           document.body.appendChild(script);
         }
-
         init();
       })();
     </script>