Fix iOS unchanged local assets refetched from packager in development

Summary:
Hi,

This PR fixes the problem described by chrisnojima in facebook/react-native#9581 (comment)

**Test plan**

In development mode,
- Run an app with an image: `<Image source={ require('./logo.png') }/>`
- Notice that you see the following in packager console:
```txt
6:46:42 PM] <START> processing asset request logo.png
[6:46:42 PM] <END>   processing asset request logo.png (1ms)
```
- Reload the app, or navigate to another page of the app with the same image
- Notice that you see again:
```txt
6:47:23 PM] <START> processing asset request logo.png
[6:47:23 PM] <END>   processing asset request logo.png (1ms)
```

Now wih the fix applied,
notice that you only see `logo.png` fetched once, even if you reload or show the same image in a different part of the app.

Let me know what you think.
Closes facebook/react-native#9795

Differential Revision: D3876945

Pulled By: davidaurelio

fbshipit-source-id: f41f4719e87644692a690123fd6e54eead9cc87d

react-packager/src/Server/__tests__/Server-test.js

@@ -337,49 +337,53 @@ describe('processRequest', () => {
   describe('/assets endpoint', () => {
     it('should serve simple case', () => {
       const req = {url: '/assets/imgs/a.png'};
-      const res = {end: jest.fn()};
+      const res = {end: jest.fn(), setHeader: jest.fn()};
 
       AssetServer.prototype.get.mockImpl(() => Promise.resolve('i am image'));
 
       server.processRequest(req, res);
       jest.runAllTimers();
+      expect(res.setHeader).toBeCalledWith('Cache-Control', 'max-age=31536000');
       expect(res.end).toBeCalledWith('i am image');
     });
 
     it('should parse the platform option', () => {
       const req = {url: '/assets/imgs/a.png?platform=ios'};
-      const res = {end: jest.fn()};
+      const res = {end: jest.fn(), setHeader: jest.fn()};
 
       AssetServer.prototype.get.mockImpl(() => Promise.resolve('i am image'));
 
       server.processRequest(req, res);
       jest.runAllTimers();
       expect(AssetServer.prototype.get).toBeCalledWith('imgs/a.png', 'ios');
+      expect(res.setHeader).toBeCalledWith('Cache-Control', 'max-age=31536000');
       expect(res.end).toBeCalledWith('i am image');
     });
 
     it('should serve range request', () => {
       const req = {url: '/assets/imgs/a.png?platform=ios', headers: {range: 'bytes=0-3'}};
-      const res = {end: jest.fn(), writeHead: jest.fn()};
+      const res = {end: jest.fn(), writeHead: jest.fn(), setHeader: jest.fn()};
       const mockData = 'i am image';
 
       AssetServer.prototype.get.mockImpl(() => Promise.resolve(mockData));
 
       server.processRequest(req, res);
       jest.runAllTimers();
       expect(AssetServer.prototype.get).toBeCalledWith('imgs/a.png', 'ios');
+      expect(res.setHeader).toBeCalledWith('Cache-Control', 'max-age=31536000');
       expect(res.end).toBeCalledWith(mockData.slice(0, 4));
     });
 
     it('should serve assets files\'s name contain non-latin letter', () => {
       const req = {url: '/assets/imgs/%E4%B8%BB%E9%A1%B5/logo.png'};
-      const res = {end: jest.fn()};
+      const res = {end: jest.fn(), setHeader: jest.fn()};
 
       AssetServer.prototype.get.mockImpl(() => Promise.resolve('i am image'));
 
       server.processRequest(req, res);
       jest.runAllTimers();
       expect(AssetServer.prototype.get).toBeCalledWith('imgs/主页/logo.png', undefined);
+      expect(res.setHeader).toBeCalledWith('Cache-Control', 'max-age=31536000');
       expect(res.end).toBeCalledWith('i am image');
     });
   });

react-packager/src/Server/index.js

@@ -493,7 +493,12 @@ class Server {
     const assetEvent = Activity.startEvent('Processing asset request', {asset: assetPath[1]});
     this._assetServer.get(assetPath[1], urlObj.query.platform)
       .then(
-        data => res.end(this._rangeRequestMiddleware(req, res, data, assetPath)),
+        data => {
+          // Tell clients to cache this for 1 year.
+          // This is safe as the asset url contains a hash of the asset.
+          res.setHeader('Cache-Control', 'max-age=31536000');
+          res.end(this._rangeRequestMiddleware(req, res, data, assetPath));
+        },
         error => {
           console.error(error.stack);
           res.writeHead('404');