Remove deprecated Strings class.

pkg/args/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:args/args.dart';
 
 main() {
@@ skipping 193 matching lines @@
     } else {
       if (line.substring(0, indent).trim() != '') {
         throw new ArgumentError(
             'Line "$line" does not have enough indentation.');
       }
 
       lines[i] = line.substring(indent);
     }
   }
 
-  return Strings.join(lines, '\n');
+  return lines.join('\n');
 }