react-devtools: set icon on macos (#21908)

packages/react-devtools/app.js

@@ -9,6 +9,7 @@
 
 const {app, BrowserWindow} = require('electron'); // Module to create native browser window.
 const {join} = require('path');
+const os = require('os');
 
 const argv = require('minimist')(process.argv.slice(2));
 const projectRoots = argv._;
@@ -32,6 +33,11 @@ app.on('ready', function() {
     },
   });
 
+  // set dock icon for macos
+  if (os.platform() === 'darwin') {
+    app.dock.setIcon(join(__dirname, 'icons/icon128.png'));
+  }
+
   // https://stackoverflow.com/questions/32402327/
   mainWindow.webContents.on('new-window', function(event, url) {
     event.preventDefault();