Let users specify the start page of the resulting docs

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 14 matching lines @@
   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(),
       introFileName: introduction,
       out: results['out'],
       excludeLibraries: excludedLibraries,
-      includeDependentPackages: results['include-dependent-packages']);
+      includeDependentPackages: results['include-dependent-packages'],
+      startPage: results['startPage']);
 }
 
 /**
  * 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 34 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: false, negatable: false);
+  parser.addOption('startPage',
+      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. --startPage=intl will make '
+        'the start page of the viewer be the intl package.',
+        defaultsTo: null);
+
   return parser;
 }