Update README.md and HACKING.md and resulting yak shave.

sky/sdk/lib/sky_tool

 #!/usr/bin/env python
 # Copyright 2015 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.
 
 import argparse
 import json
 import logging
 import os
 import re
@@ skipping 22 matching lines @@
     'sky_server_root',
     'build_dir',
 ])
 
 
 def _port_in_use(port):
     sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
     return sock.connect_ex(('localhost', port)) == 0
 
 
-# We need something to serve sky files, python's httpserver is sufficient.
+# We need something to serve dart files, python's httpserver is sufficient.
 def _start_http_server(port, root):
     server_command = [
         'python', '-m', 'SimpleHTTPServer', str(port),
     ]
     return subprocess.Popen(server_command, cwd=root).pid
 
 
 # This 'strict dictionary' approach is useful for catching typos.
 class Pids(object):
     def __init__(self, known_keys, contents=None):
@@ skipping 56 matching lines @@
     relative_path = os.path.relpath(path, root)
     return 'sky://localhost:%s/%s' % (port, relative_path)
 
 
 class StartSky(object):
     def add_subparser(self, subparsers):
         start_parser = subparsers.add_parser('start',
             help='launch %s on the device' % APK_NAME)
         start_parser.add_argument('--install', action='store_true')
         start_parser.add_argument('project_or_path', nargs='?', type=str,
-            default='main.sky')
+            default='main.dart')
         start_parser.set_defaults(func=self.run)
 
     def _is_package_installed(self, package_name):
         pm_path_cmd = [ADB_PATH, 'shell', 'pm', 'path', package_name]
         return subprocess.check_output(pm_path_cmd).strip() != ''
 
     def run(self, args, pids):
         StopSky().run(args, pids)
 
         if not self._is_package_installed(ANDROID_PACKAGE):
             print '%s is not installed, installing.' % APK_NAME
             args.install = True
 
         if args.install:
             apk_path = os.path.join(SKY_PACKAGE_ROOT, 'apks', APK_NAME)
             if not os.path.exists(apk_path):
                 print "'%s' does not exist?" % apk_path
                 return 2
 
             subprocess.check_call([ADB_PATH, 'install', '-r', apk_path])
 
         project_or_path = os.path.abspath(args.project_or_path)
 
         if os.path.isdir(project_or_path):
             sky_server_root = project_or_path
-            main_sky = os.path.join(project_or_path, 'main.sky')
-            missing_msg = "Missing main.sky in project: %s" % sky_server_root
+            main_sky = os.path.join(project_or_path, 'main.dart')
+            missing_msg = "Missing main.dart in project: %s" % sky_server_root
         else:
             sky_server_root = os.path.dirname(project_or_path)
             main_sky = project_or_path
             missing_msg = "%s does not exist." % main_sky
 
         if not os.path.isfile(main_sky):
             print missing_msg
             return 2
 
         sky_server_port = SKY_SERVER_PORT
@@ skipping 120 matching lines @@
         args = parser.parse_args()
         pids = Pids.read_from(PID_FILE_PATH, PID_FILE_KEYS)
         exit_code = args.func(args, pids)
         # We could do this with an at-exit handler instead?
         pids.write_to(PID_FILE_PATH)
         sys.exit(exit_code)
 
 
 if __name__ == '__main__':
     SkyShellRunner().main()