Allow defining a help string for an option's parameter value.

pkg/args/lib/args.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;
 
 import 'package:collection/wrappers.dart';
 
 import 'src/parser.dart';
 import 'src/usage.dart';
@@ skipping 50 matching lines @@
     _commands[name] = parser;
     return parser;
   }
 
   /// Defines a flag. Throws an [ArgumentError] if:
   ///
   /// * There is already an option named [name].
   /// * There is already an option using abbreviation [abbr].
   void addFlag(String name, {String abbr, String help, bool defaultsTo: false,
       bool negatable: true, void callback(bool value), bool hide: false}) {
-    _addOption(name, abbr, help, null, null, defaultsTo, callback,
+    _addOption(name, abbr, help, null, null, null, defaultsTo, callback,
         isFlag: true, negatable: negatable, hide: hide);
   }
 
   /// Defines a value-taking option. Throws an [ArgumentError] if:
   ///
   /// * There is already an option with name [name].
   /// * There is already an option using abbreviation [abbr].
-  void addOption(String name, {String abbr, String help, List<String> allowed,
-      Map<String, String> allowedHelp, String defaultsTo,
+  void addOption(String name, {String abbr, String help, String valueHelp,
+      List<String> allowed, Map<String, String> allowedHelp, String defaultsTo,
       void callback(value), bool allowMultiple: false, bool hide: false}) {
-    _addOption(name, abbr, help, allowed, allowedHelp, defaultsTo,
+    _addOption(name, abbr, help, valueHelp, allowed, allowedHelp, defaultsTo,
         callback, isFlag: false, allowMultiple: allowMultiple,
         hide: hide);
   }
 
-  void _addOption(String name, String abbr, String help, List<String> allowed,
-      Map<String, String> allowedHelp, defaultsTo,
+  void _addOption(String name, String abbr, String help, String valueHelp,
+      List<String> allowed, Map<String, String> allowedHelp, defaultsTo,
       void callback(value), {bool isFlag, bool negatable: false,
       bool allowMultiple: false, bool hide: false}) {
     // Make sure the name isn't in use.
     if (_options.containsKey(name)) {
       throw new ArgumentError('Duplicate option "$name".');
     }
 
     // Make sure the abbreviation isn't too long or in use.
     if (abbr != null) {
       var existing = findByAbbreviation(abbr);
       if (existing != null) {
         throw new ArgumentError(
             'Abbreviation "$abbr" is already used by "${existing.name}".');
       }
     }
 
-    _options[name] = new Option(name, abbr, help, allowed, allowedHelp,
-        defaultsTo, callback, isFlag: isFlag, negatable: negatable,
+    _options[name] = new Option(name, abbr, help, valueHelp, allowed,
+        allowedHelp, defaultsTo, callback, isFlag: isFlag, negatable: negatable,
         allowMultiple: allowMultiple, hide: hide);
   }
 
   /// Parses [args], a list of command-line arguments, matches them against the
   /// flags and options defined by this parser, and returns the result.
   ArgResults parse(List<String> args) =>
       new Parser(null, this, args.toList(), null, null).parse();
 
   /// Generates a string displaying usage information for the defined options.
   ///
@@ skipping 53 matching lines @@
           'Could not find an option named "$name".');
     }
 
     return _options[name];
   }
 
   /// Get the names of the options as an [Iterable].
   Iterable<String> get options => _options.keys;
 }