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

Libraries/BatchedBridge/BatchedBridge.js

@@ -25,6 +25,11 @@ const JSTimersExecution = require('JSTimersExecution');
 BatchedBridge.registerCallableModule('Systrace', Systrace);
 BatchedBridge.registerCallableModule('JSTimersExecution', JSTimersExecution);
 
+if (__DEV__) {
+  const HMRClient = require('HMRClient');
+  BatchedBridge.registerCallableModule('HMRClient', HMRClient);
+}
+
 // Wire up the batched bridge on the global object so that we can call into it.
 // Ideally, this would be the inverse relationship. I.e. the native environment
 // provides this global directly with its script embedded. Then this module

Libraries/Utilities/HMRClient.js

@@ -0,0 +1,40 @@
+/**
+ * Copyright (c) 2015-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ *
+ * @providesModule HMRClient
+ */
+'use strict';
+
+let _activeWS;
+
+/**
+ * HMR Client that receives from the server HMR updates and propagates them
+ * runtime to reflects those changes.
+ */
+const HMRClient = {
+  setEnabled(enabled) {
+    if (_activeWS && _activeWS) {
+      _activeWS.close();
+      _activeWS = null;
+    }
+
+    if (enabled) {
+      // TODO(martinb): parametrize the url and receive entryFile to minimize 
+      // the number of updates we want to receive from the server.
+      _activeWS = new WebSocket('ws://localhost:8081/hot');
+      _activeWS.onerror = (e) => {
+        console.error('[Hot Module Replacement] Unexpected error', e);
+      };
+      _activeWS.onmessage = (m) => {
+        // TODO(martinb): inject HMR update
+      };
+    }
+  },
+};
+
+module.exports = HMRClient;