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

sky/sdk/lib/sky_tool

Index: sky/sdk/lib/sky_tool
diff --git a/sky/sdk/lib/sky_tool b/sky/sdk/lib/sky_tool
index 1dfe31719aba5a4539e922d23c178571b534c015..cbdbbd6c771d677a11f29dbd1b890f858a1a15f8 100755
--- a/sky/sdk/lib/sky_tool
+++ b/sky/sdk/lib/sky_tool
@@ -40,7 +40,7 @@ def _port_in_use(port):
     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),
@@ -117,7 +117,7 @@ class StartSky(object):
             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):
@@ -143,8 +143,8 @@ class StartSky(object):
 
         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