Tail

Summary:
Added a new type, Tail. It observes changes made to a file and notifies the subscriber.

It is useful and used to tail flipper-server logs, for example.

Reviewed By: passy

Differential Revision: D36473529

fbshipit-source-id: b0f905f4f8419d9e13386440d2d2cdabd528295b

desktop/flipper-frontend-core/src/index.tsx

@@ -15,3 +15,4 @@ export * from './globalObject';
 export * from './plugins';
 export * from './flipperLibImplementation';
 export * from './client/FlipperServerClient';
+export * from './utils/tail';