Automatically pass SDK into docgen.dart, and use the SDK for running

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';
@@ skipping 17 matching lines @@
   var startPage = options['start-page'];
   if (_singlePackage(_files) && startPage == null) {
     startPage = _defaultStartPage;
     print("Using default options for documenting a single package: "
         "--start-page=$startPage");
   }
   var includeSdk = options['parse-sdk'] || options['include-sdk'];
   var scriptDir = path.dirname(Platform.script.toFilePath());
   var introduction = includeSdk ? '' : options['introduction'];
 
+  var pubScript = options['sdk'] != null ? 
+      path.join(options['sdk'], 'bin', 'pub') : 'pub';
+
+  var dartBinary = options['sdk'] != null ? 
+      path.join(options['sdk'], 'bin', 'dart') : 'dart';
+
   docgen(_files,
       packageRoot: options['package-root'],
       outputToYaml: !options['json'],
       includePrivate: options['include-private'],
       includeSdk: includeSdk,
       parseSdk: options['parse-sdk'],
       append: options['append'] && new Directory(options['out']).existsSync(),
       introFileName: introduction,
       out: options['out'],
       excludeLibraries: excludedLibraries,
       includeDependentPackages: options['include-dependent-packages'],
       serve: options['serve'],
+      dartBinary: dartBinary,
+      pubScript: pubScript,
       noDocs: options['no-docs'],
       startPage: startPage);
 }
 
 /**
  * Print help if we are passed the help option or invalid arguments.
  */
 void _printHelpAndExit() {
   print(_initArgParser().getUsage());
   print('Usage: dart docgen.dart [OPTIONS] fooDir/barFile');
@@ skipping 69 matching lines @@
       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: true, negatable: true);
+  parser.addOption('sdk',
+      help: 'SDK directory',
+      defaultsTo: null);
   parser.addOption('start-page',
       help: 'By default the viewer will start at the SDK introduction page.'
         'To start at some other page, e.g. for a package, provide the name '
         'of the package in this argument, e.g. --start-page=intl will make '
         'the start page of the viewer be the intl package.',
         defaultsTo: null);
 
   return parser;
 }