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

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';
 
 void main() {
@@ skipping 104 matching lines @@
       var parser = new ArgParser();
       parser.addOption('any', help: 'Can be anything', defaultsTo: 'whatevs');
 
       validateUsage(parser,
           '''
           --any    Can be anything
                    (defaults to "whatevs")
           ''');
     });
 
+    test('the value help is shown', () {
+      var parser = new ArgParser();
+      parser.addOption('out', abbr: 'o', help: 'Where to write file',
+          valueHelp: 'path');
+
+      validateUsage(parser,
+          '''
+          -o, --out=<path>    Where to write file
+          ''');
+    });
+
     test('the allowed list is shown', () {
       var parser = new ArgParser();
       parser.addOption('suit', help: 'Like in cards',
           allowed: ['spades', 'clubs', 'hearts', 'diamonds']);
 
       validateUsage(parser,
           '''
           --suit    Like in cards
                     [spades, clubs, hearts, diamonds]
           ''');
@@ skipping 109 matching lines @@
         throw new ArgumentError(
             'Line "$line" does not have enough indentation.');
       }
 
       lines[i] = line.substring(indent);
     }
   }
 
   return lines.join('\n');
 }