RN: Support Multiple Packager Servers

Summary:
Adds support for multiple packager servers, which could be used to hot-reload dependencies in response to a change in the `cacheVersion`.

Currently, there is no way to:

- Create multiple `ReactPackager` servers.
- Instantiate more than one `FileWatcher` constructor (due to the "single instance" invariant).

public

Reviewed By: martinbigio

Differential Revision: D2713455

fb-gh-sync-id: 9be0f0cb2b846baf088d0cf14650cc8b9e950815

packager/react-packager/index.js

@@ -16,6 +16,7 @@ var debug = require('debug');
 var omit = require('underscore').omit;
 var Activity = require('./src/Activity');
 
+exports.createServer = createServer;
 exports.middleware = function(options) {
   var server = createServer(options);
   return server.processRequest.bind(server);

packager/react-packager/src/FileWatcher/index.js

@@ -72,6 +72,7 @@ class FileWatcher extends EventEmitter {
   }
 
   end() {
+    inited = false;
     return this._loading.then(
       (watchers) => watchers.map(
         watcher => Promise.denodeify(watcher.close).call(watcher)