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

pkg/docgen/bin/docgen.dart

Index: pkg/docgen/bin/docgen.dart
===================================================================
--- pkg/docgen/bin/docgen.dart	(revision 32622)
+++ pkg/docgen/bin/docgen.dart	(working copy)
@@ -35,6 +35,12 @@
   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'],
@@ -47,6 +53,8 @@
       excludeLibraries: excludedLibraries,
       includeDependentPackages: options['include-dependent-packages'],
       serve: options['serve'],
+      dartBinary: dartBinary,
+      pubScript: pubScript,
       noDocs: options['no-docs'],
       startPage: startPage);
 }
@@ -136,6 +144,9 @@
         '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 '