Make package: work like Dart expects in preparation for a Sky SDK

sky/tools/skygo/sky_server.go

 // Copyright (c) 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 package main
 
 import (
     "flag"
     "fmt"
     "io/ioutil"
@@ skipping 37 matching lines @@
 
 func main() {
     var configuration = flag.String("t", "Release", "The target configuration (i.e. Release or Debug)")
 
     flag.Parse()
     flag.Usage = usage
     // The built-in go:flag is awful.  It only allows short-name arguments
     // and they *must* be before any unnamed arguments.  There are better ones:
     // https://godoc.org/github.com/jessevdk/go-flags
     // but for now we're using raw-go.
-    if flag.NArg() != 2 {
+    if flag.NArg() != 3 {
         usage()
     }
 
     root, _ := filepath.Abs(flag.Arg(0))
     port := flag.Arg(1)
+    packageRoot := flag.Arg(2)
 
+    // genRoot should not be needed once we figure out how mojom generation
+    // for sdk users should work.
     genRoot := path.Join(root, "out", *configuration, "gen")
 
     fmt.Fprintf(os.Stderr, "Mappings for localhost:%s:\n", port)
 
     fmt.Fprintf(os.Stderr, "  / -> %s\n", root)
     http.Handle("/", skyHandler(root))
 
     fmt.Fprintf(os.Stderr, "  /echo_post -> custom echo handler\n")
     http.HandleFunc("/echo_post", func(w http.ResponseWriter, r *http.Request) {
         defer r.Body.Close()
         body, _ := ioutil.ReadAll(r.Body)
         w.Write(body)
     })
 
     addMapping("/gen/", genRoot)
+    addMapping("/packages/", packageRoot)
 
     http.ListenAndServe(":" + port, nil)
 }