feat(Response): add Response.fromCache / Response.fromServiceWorker (#1971)

This patch:
- introduces `test/assets/cached` folder and teaches server to cache
  all the assets from the folder
- introduces `test/assets/serviceworkers` folder that stores all the
  service workers and makes them register with unique URL prefix
- introduces `Response.fromCache()` and `Response.fromServiceWorker()`
  methods

Fixes #1551.

test/server/SimpleServer.js

@@ -68,6 +68,9 @@ class SimpleServer {
     this._server.listen(port);
     this._dirPath = dirPath;
 
+    this._startTime = new Date();
+    this._cachedPathPrefix = null;
+
     /** @type {!Set<!net.Socket>} */
     this._sockets = new Set();
 
@@ -90,6 +93,13 @@ class SimpleServer {
     socket.once('close', () => this._sockets.delete(socket));
   }
 
+  /**
+   * @param {string} pathPrefix
+   */
+  enableHTTPCache(pathPrefix) {
+    this._cachedPathPrefix = pathPrefix;
+  }
+
   /**
    * @param {string} path
    * @param {string} username
@@ -189,15 +199,27 @@ class SimpleServer {
     let pathName = url.parse(request.url).path;
     if (pathName === '/')
       pathName = '/index.html';
-    pathName = path.join(this._dirPath, pathName.substring(1));
+    const filePath = path.join(this._dirPath, pathName.substring(1));
 
-    fs.readFile(pathName, function(err, data) {
-      if (err) {
-        response.statusCode = 404;
-        response.end(`File not found: ${pathName}`);
+    if (this._cachedPathPrefix !== null && filePath.startsWith(this._cachedPathPrefix)) {
+      if (request.headers['if-modified-since']) {
+        response.statusCode = 304; // not modified
+        response.end();
         return;
       }
-      response.setHeader('Content-Type', mime.lookup(pathName));
+      response.setHeader('Cache-Control', 'public, max-age=31536000');
+      response.setHeader('Last-Modified', this._startTime.toString());
+    } else {
+      response.setHeader('Cache-Control', 'no-cache, no-store');
+    }
+
+    fs.readFile(filePath, function(err, data) {
+      if (err) {
+        response.statusCode = 404;
+        response.end(`File not found: ${filePath}`);
+        return;
+      }
+      response.setHeader('Content-Type', mime.lookup(filePath));
       response.end(data);
     });
   }