fix: traivs dart version to 2.4.0

Author: xuzhongpeng

.travis.yml

@@ -1,7 +1,7 @@
 language: dart
 
 dart:
-  - 2.2.0
+  - 2.4.0
   - dev
 
 dart_task:

lib/utils/logging.dart

@@ -1,9 +1,5 @@
-// Copyright (c) 2019, the Dart project authors.  Please see the AUTHORS file
-// for details. All rights reserved. Use of this source code is governed by a
-// BSD-style license that can be found in the LICENSE file.
+// ignore_for_file: public_member_api_docs
 
-/// This library contains functionality to help command-line utilities to easily
-/// create aesthetic output.
 
 import 'dart:async';
 import 'dart:io' as io;

lib/utils/type.dart

@@ -1,4 +1,7 @@
+// ignore_for_file: public_member_api_docs
+
 /// all git hooks from [git hooks documentation](https://git-scm.com/docs/githooks.html)
+
 enum Git {
   applypatchMsg,
   preApplypatch,