Add a snapshot for docgen and use it in the build

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;
 
 List<String> excludedLibraries = [];
 
 /**
  * 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);
 
   var includeSdk = results['parse-sdk'] || results['include-sdk'];
   var scriptDir = path.dirname(Platform.script.toFilePath());
-  var introFile =
-      path.join(path.dirname(scriptDir), 'doc', 'sdk-introduction.md');
-  var introduction = includeSdk ? introFile : results['introduction'];
+  var introduction = includeSdk ? '' : results['introduction'];
   docgen(results.rest.map(path.normalize).toList(),
       packageRoot: results['package-root'],
       outputToYaml: !results['json'],
       includePrivate: results['include-private'],
       includeSdk: includeSdk,
       parseSdk: results['parse-sdk'],
       append: results['append'] && new Directory(results['out']).existsSync(),
-      introduction: introduction,
+      introFileName: introduction,
       out: results['out'],
       excludeLibraries: excludedLibraries,
       includeDependentPackages: results['include-dependent-packages']);
 }
 
 /**
  * Creates parser for docgen command line arguments.
  */
 ArgParser _initArgParser() {
   var parser = new ArgParser();
@@ skipping 24 matching lines @@
   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.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: '');
+        ' for the generated documentation.', defaultsTo: '');
   parser.addOption('out',
       help: 'The name of the output directory.',
       defaultsTo: 'docs');
   parser.addOption('exclude-lib',
       help: 'Exclude the library by this name from the documentation',
       allowMultiple: true,
       callback: (libs) => excludedLibraries.addAll(libs));
   parser.addFlag('include-dependent-packages',
       help: 'Assumes we are documenting a single package and are running '
         'in the directory with its pubspec. Includes documentation for all '
         'of its dependent packages.',
       defaultsTo: false, negatable: false);
   return parser;
 }