Roll Observatory packages and add a roll script

packages/args/lib/src/arg_parser.dart

 // 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.src.arg_parser;
 
 import 'dart:collection';
 
 import 'arg_results.dart';
 import 'option.dart';
@@ skipping 143 matching lines @@
     return options[option].defaultValue;
   }
 
   /// Finds the option whose abbreviation is [abbr], or `null` if no option has
   /// that abbreviation.
   Option findByAbbreviation(String abbr) {
     return options.values.firstWhere((option) => option.abbreviation == abbr,
         orElse: () => null);
   }
 }