Let users specify the start page of the resulting docs

pkg/docgen/bin/docgen.dart

Index: pkg/docgen/bin/docgen.dart
diff --git a/pkg/docgen/bin/docgen.dart b/pkg/docgen/bin/docgen.dart
index 2a116de5b1bec5557663576bbb3959b1757c1734..86d4c9eb62653cb4fa7ae855c90d8e561d456821 100644
--- a/pkg/docgen/bin/docgen.dart
+++ b/pkg/docgen/bin/docgen.dart
@@ -32,7 +32,8 @@ void main(List<String> arguments) {
       introFileName: introduction,
       out: results['out'],
       excludeLibraries: excludedLibraries,
-      includeDependentPackages: results['include-dependent-packages']);
+      includeDependentPackages: results['include-dependent-packages'],
+      startPage: results['startPage']);
 }
 
 /**
@@ -87,5 +88,12 @@ ArgParser _initArgParser() {
         '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;
 }