Introduce Packager and App HMR WebSocket connection

Summary:
public

This diff adds infra to both the Packager and the running app to have a WebSocket based connection between them. This connection is toggled by a new dev menu item, namely `Enable/Disable Hot Loading`.

Reviewed By: vjeux

Differential Revision: D2787621

fb-gh-sync-id: d1dee769348e4830c28782e7b650d025f2b3a786

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

@@ -134,6 +134,7 @@ class Server {
     this._projectRoots = opts.projectRoots;
     this._bundles = Object.create(null);
     this._changeWatchers = [];
+    this._fileChangeListeners = [];
 
     const assetGlobs = opts.assetExts.map(ext => '**/*.' + ext);
 
@@ -175,6 +176,7 @@ class Server {
     this._fileWatcher.on('all', this._onFileChange.bind(this));
 
     this._debouncedFileChangeHandler = _.debounce(filePath => {
+      this._fileChangeListeners.forEach(listener => listener(filePath));
       this._rebuildBundles(filePath);
       this._informChangeWatchers();
     }, 50);
@@ -187,6 +189,10 @@ class Server {
     ]);
   }
 
+  addFileChangeListener(listener) {
+    this._fileChangeListeners.push(listener);
+  }
+
   buildBundle(options) {
     return Promise.resolve().then(() => {
       if (!options.platform) {