Don't disconnect DevTools WebSocket connection on Cmd+D

Summary:
When the React Native Cmd+D menu is opened, something re-runs module initialization code (including DevTools backend initialization) which recreates the `WebSocket` and kills any already-connected frontend.

It's not clear to me why this is done. (Intentional? Accident?) But it makes it difficult to connect the React Native Inspector and DevTools together without multiple reloads. This Diff prevents the Cmd+D menu from killing the `WebSocket` connection as a workaround. A better long-term fix would (probably) be to not eagerly re-run these modules.

## Changelog

[General] [Fixed] - Don't disconnect DevTools WebSocket connection on Cmd+D

Reviewed By: fkgozali, sammy-SC

Differential Revision: D27742376

fbshipit-source-id: 60ab3e4763da6b055c28c7aafc6d460e7f4a601d

Libraries/Core/setUpReactDevTools.js

@@ -11,8 +11,19 @@
 'use strict';
 
 if (__DEV__) {
+  let isWebSocketOpen = false;
+  let ws = null;
+
   const reactDevTools = require('react-devtools-core');
   const connectToDevTools = () => {
+    if (ws !== null && isWebSocketOpen) {
+      // If the DevTools backend is already connected, don't recreate the WebSocket.
+      // This would break the connection.
+      // If there isn't an active connection, a backend may be waiting to connect,
+      // in which case it's okay to make a new one.
+      return;
+    }
+
     // not when debugging in chrome
     // TODO(t12832058) This check is broken
     if (!window.document) {
@@ -39,7 +50,13 @@ if (__DEV__) {
           : 8097;
 
       const WebSocket = require('../WebSocket/WebSocket');
-      const ws = new WebSocket('ws://' + host + ':' + port);
+      ws = new WebSocket('ws://' + host + ':' + port);
+      ws.addEventListener('close', event => {
+        isWebSocketOpen = false;
+      });
+      ws.addEventListener('open', event => {
+        isWebSocketOpen = true;
+      });
 
       const viewConfig = require('../Components/View/ReactNativeViewViewConfig');
       reactDevTools.connectToDevTools({