Upgrade to test 0.12.0.

test/usage_test.dart

 // Copyright (c) 2012, 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 usage_test;
 
-import 'package:unittest/unittest.dart';
+import 'package:test/test.dart';
 import 'package:args/args.dart';
 
 void main() {
   group('ArgParser.usage', () {
     test('negatable flags show "no-" in title', () {
       var parser = new ArgParser();
       parser.addFlag('mode', help: 'The mode');
 
       validateUsage(parser, '''
           --[no-]mode    The mode
@@ skipping 222 matching lines @@
         throw new ArgumentError(
             'Line "$line" does not have enough indentation.');
       }
 
       lines[i] = line.substring(indent);
     }
   }
 
   return lines.join('\n');
 }