Roll Observatory packages and add a roll script

packages/args/lib/src/parser.dart

 // Copyright (c) 2013, 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.src.parser;
 
 import 'arg_parser.dart';
 import 'arg_results.dart';
 import 'option.dart';
 
@@ skipping 269 matching lines @@
   }
 
   /// Validates that [value] is allowed as a value of [option].
   void _validateAllowed(Option option, String value) {
     if (option.allowed == null) return;
 
     validate(option.allowed.contains(value),
         '"$value" is not an allowed value for option "${option.name}".');
   }
 }