Stopgap generating docgen all at once until --append mode can be fixed

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';
 import 'package:path/path.dart' as path;
 
 /**
  * Analyzes Dart files and generates a representation of included libraries,
  * classes, and members.
  */
 void main(List<String> arguments) {
   logger.onRecord.listen((record) => print(record.message));
   var results = _initArgParser().parse(arguments);
 
   docgen(results.rest.map(path.normalize).toList(),
       packageRoot: results['package-root'],
       outputToYaml: !results['json'],
       includePrivate: results['include-private'],
       includeSdk: results['parse-sdk'] || results['include-sdk'],
       parseSdk: results['parse-sdk'],
       append: results['append'] && new Directory('docs').existsSync(),
-      introduction: results['parse-sdk'] ?
+      introduction: (results['parse-sdk'] || results['include-sdk']) ?
           'sdk-introduction.md' : results['introduction']);
 }
 
 /**
  * Creates parser for docgen command line arguments.
  */
 ArgParser _initArgParser() {
   var parser = new ArgParser();
   parser.addFlag('help', abbr: 'h',
       help: 'Prints help and usage information.',
@@ skipping 26 matching lines @@
       help: 'Sets the package root of the library being analyzed.');
   parser.addFlag('append',
       help: 'Append to the docs folder, library_list.json and index.txt',
       defaultsTo: false, negatable: false);
   parser.addOption('introduction',
       help: 'Adds the provided markdown text file as the introduction'
         ' for the outputted documentation.', defaultsTo: '');
 
   return parser;
 }