Re-add bin/ to flutter_plugin_tools (#3839)

This should have been re-added in #3836 but was missed, so it doesn't
work correctly via `pub global`.

script/tool/CHANGELOG.md

@@ -1,3 +1,7 @@
+## 0.1.0+1
+
+- Re-add the bin/ directory.
+
 ## 0.1.0
 
 - **NOTE**: This is no longer intended as a general-purpose package, and is now

script/tool/bin/flutter_plugin_tools.dart

@@ -0,0 +1,5 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+export 'package:flutter_plugin_tools/src/main.dart';

script/tool/pubspec.yaml

@@ -1,7 +1,7 @@
 name: flutter_plugin_tools
 description: Productivity utils for flutter/plugins and flutter/packages
 repository: https://github.com/flutter/plugins/tree/master/script/tool
-version: 0.1.0
+version: 0.1.0+1
 
 dependencies:
   args: "^1.4.3"