fix: devtools cannot be closed correctly (facebook#25510)

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## Summary

Fix devtools cannot be shutdown by bridge.shutdown().

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Author: Jack-Works

packages/react-devtools-shared/src/__tests__/bridge-test.js

@@ -21,6 +21,8 @@ describe('Bridge', () => {
       send: jest.fn(),
     };
     const bridge = new Bridge(wall);
+    const shutdownCallback = jest.fn();
+    bridge.addListener('shutdown', shutdownCallback);
 
     // Check that we're wired up correctly.
     bridge.send('reloadAppForProfiling');
@@ -36,6 +38,7 @@ describe('Bridge', () => {
     expect(wall.send).toHaveBeenCalledWith('update', '1');
     expect(wall.send).toHaveBeenCalledWith('update', '2');
     expect(wall.send).toHaveBeenCalledWith('shutdown');
+    expect(shutdownCallback).toHaveBeenCalledTimes(1);
 
     // Verify that the Bridge doesn't send messages after shutdown.
     jest.spyOn(console, 'warn').mockImplementation(() => {});

packages/react-devtools-shared/src/bridge.js

@@ -334,6 +334,7 @@ class Bridge<
     }
 
     // Queue the shutdown outgoing message for subscribers.
+    this.emit('shutdown');
     this.send('shutdown');
 
     // Mark this bridge as destroyed, i.e. disable its public API.