Add support for hidden options to pkg/args.

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 149 matching lines @@
       validateUsage(parser,
           '''
           --suit              Like in cards
 
                 [clubs]       Weapons of war
                 [diamonds]    Money for this art
                 [hearts]      The shape of my heart
                 [spades]      Swords of a soldier
           ''');
     });
+
+    test("hidden flags don't appear in the help", () {
+      var parser = new ArgParser();
+      parser.addOption('first', help: 'The first option');
+      parser.addOption('second', isHidden: true);
+      parser.addOption('third', help: 'The third option');
+
+
+      validateUsage(parser,
+          '''
+          --first     The first option
+          --third     The third option
+          ''');
+    });
   });
 }
 
 throwsIllegalArg(function) {
   expect(function, throwsArgumentError);
 }
 
 validateUsage(ArgParser parser, String expected) {
   expected = unindentString(expected);
   expect(parser.getUsage(), equals(expected));
@@ skipping 26 matching lines @@
         throw new ArgumentError(
             'Line "$line" does not have enough indentation.');
       }
 
       lines[i] = line.substring(indent);
     }
   }
 
   return lines.join('\n');
 }