Add support for hidden options to pkg/args.

pkg/args/lib/src/usage.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.usage;
 
 import 'dart:math';
 
 import '../args.dart';
 
@@ skipping 49 matching lines @@
   /**
    * Generates a string displaying usage information for the defined options.
    * This is basically the help text shown on the command line.
    */
   String generate() {
     buffer = new StringBuffer();
 
     calculateColumnWidths();
 
     args.options.forEach((name, option) {
+      if (option.isHidden) return;
+
       write(0, getAbbreviation(option));
       write(1, getLongOption(option));
 
       if (option.help != null) write(2, option.help);
 
       if (option.allowedHelp != null) {
         var allowedNames = option.allowedHelp.keys.toList(growable: false);
         allowedNames.sort();
         newline();
         for (var name in allowedNames) {
@@ skipping 149 matching lines @@
 String padRight(String source, int length) {
   final result = new StringBuffer();
   result.write(source);
 
   while (result.length < length) {
     result.write(' ');
   }
 
   return result.toString();
 }