Let docgen detect a dart-sdk directory that's differently named, and do it correctly on bleeding ed…

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 'package:path/path.dart' as path;
 
@@ skipping 37 matching lines @@
       introFileName: introduction,
       out: options['out'],
       excludeLibraries: excludedLibraries,
       includeDependentPackages: options['include-dependent-packages'],
       compile: options['compile'],
       serve: options['serve'],
       dartBinary: dartBinary,
       pubScript: pubScript,
       noDocs: options['no-docs'],
       startPage: startPage,
-      indentJSON: indentJSON);
+      indentJSON: indentJSON,
+      sdk: options['sdk']);
 }
 
 /**
  * Print help if we are passed the help option or invalid arguments.
  */
 void _printHelpAndExit() {
   print(_initArgParser().usage);
   print('Usage: dartdocgen [OPTIONS] fooDir/barFile');
   exit(0);
 }
@@ skipping 77 matching lines @@
         '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);
   parser.addFlag('indent-json',
       help: 'Indents each level of JSON output by two spaces',
       defaultsTo: false, negatable: true);
 
   return parser;
 }