Added a flag --append to docgen.

pkg/docgen/bin/docgen.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.
 
 import 'dart:io';
 
 import 'package:args/args.dart';
 import 'package:logging/logging.dart';
 
 import '../lib/docgen.dart';
 
 /**
  * Analyzes Dart files and generates a representation of included libraries, 
  * classes, and members. 
  */
 void main() {
   logger.onRecord.listen((record) => print(record.message));
   var results = _initArgParser().parse(new Options().arguments);
   
   docgen(results.rest, 
       packageRoot: results['package-root'], 
       outputToYaml: !results['json'],  
       includePrivate: results['include-private'], 
       includeSdk: results['parse-sdk'] || results['include-sdk'], 
-      parseSdk: results['parse-sdk']);
+      parseSdk: results['parse-sdk'],
+      append: results['append'] && new Directory('docs').existsSync());
 }
 
 /**
  * Creates parser for docgen command line arguments. 
  */
 ArgParser _initArgParser() {
   var parser = new ArgParser();
   parser.addFlag('help', abbr: 'h', 
       help: 'Prints help and usage information.', 
       negatable: false, 
@@ skipping 14 matching lines @@
       negatable: true);
   parser.addFlag('include-private', 
       help: 'Flag to include private declarations.', negatable: false);
   parser.addFlag('include-sdk', 
       help: 'Flag to parse SDK Library files.', negatable: false);
   parser.addFlag('parse-sdk', 
       help: 'Parses the SDK libraries only.', 
       defaultsTo: false, negatable: false);
   parser.addOption('package-root', 
       help: "Sets the package root of the library being analyzed.");
+  parser.addFlag('append', 
+      help: 'Append to the docs folder, library_list.txt and index.txt', 
+      defaultsTo: false, negatable: false);
   
   return parser;
 }