Add observatory_pub_packages snapshot to third_party

observatory_pub_packages/args/src/usage_exception.dart

Index: observatory_pub_packages/args/src/usage_exception.dart
===================================================================
--- observatory_pub_packages/args/src/usage_exception.dart	(revision 0)
+++ observatory_pub_packages/args/src/usage_exception.dart	(working copy)
@@ -0,0 +1,14 @@
+// Copyright (c) 2014, 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.
+
+library args.usage_exception;
+
+class UsageException implements Exception {
+  final String message;
+  final String usage;
+
+  UsageException(this.message, this.usage);
+
+  String toString() => "$message\n\n$usage";
+}