feat：yoyo-getfastmirror(dart)

yoyo-getfastmirror的dart版本，易扩展

yoyo-getfastmirror/bin/yoyo_get_fast_mirror.dart

@@ -0,0 +1,18 @@
+import 'dart:io';
+
+import 'package:yoyo_get_fast_mirror/src/command_runner.dart';
+
+Future<void> main(List<String> args) async {
+  await _flushThenExit(await YoyoGetFastMirrorCommandRunner().run(args));
+}
+
+/// Flushes the stdout and stderr streams, then exits the program with the given
+/// status code.
+///
+/// This returns a Future that will never complete, since the program will have
+/// exited already. This is useful to prevent Future chains from proceeding
+/// after you've decided to exit.
+Future<void> _flushThenExit(int status) {
+  return Future.wait<void>([stdout.close(), stderr.close()])
+      .then<void>((_) => exit(status));
+}