Upgrading flow and fixing lint errors

Summary:
Upgrading to flow 0.91, fixing a bunch of `$FloxFixMe`s that were introduced by upgrading to 0.86.
Also fixing some linting issues.

Reviewed By: priteshrnandgaonkar

Differential Revision: D13900794

fbshipit-source-id: 5d0a1b62371f3b5d34b909bae0876583acb6f977

src/plugins/shared_preferences/index.js

@@ -104,7 +104,7 @@ export default class extends FlipperPlugin<SharedPreferencesState> {
       const change = event.change;
       const entry = state.sharedPreferences[change.preferences];
       if (entry == null) {
-        return;
+        return state;
       }
       if (change.deleted) {
         delete entry.preferences[change.name];