Run pub update on the pulled down version of dartdoc viewer to run.

pkg/docgen/lib/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.
 
 /// **docgen** is a tool for creating machine readable representations of Dart
 /// code metadata, including: classes, members, comments and annotations.
 ///
 /// docgen is run on a `.dart` file or a directory containing `.dart` files.
 ///
 ///      $ dart docgen.dart [OPTIONS] [FILE/DIR]
@@ skipping 643 matching lines @@
         print('Error cloning git repository:');
         print('process output: ${processResult.stdout}');
         print('process stderr: ${processResult.stderr}');
       }
     }
   }
 
   /// Move the generated json/yaml docs directory to the dartdoc-viewer
   /// directory, to run as a webpage.
   static void _moveDirectoryAndServe() {
+    var processResult = Process.runSync('pub', ['update'], runInShell: true,
+        workingDirectory: path.join(_dartdocViewerDir.path, 'client'));
+    print('process output: ${processResult.stdout}');
+    print('process stderr: ${processResult.stderr}');
+
     var dir = new Directory(_Generator._outputDirectory == null? 'docs' :
         _Generator._outputDirectory);
     var webDocsDir = new Directory(path.join(_dartdocViewerDir.path, 'client',
         'web', 'docs'));
     if (dir.existsSync()) {
       // Move the docs folder to dartdoc-viewer/client/web/docs
       dir.renameSync(webDocsDir.path);
     }
 
     if (webDocsDir.existsSync()) {
@@ skipping 1460 matching lines @@
         .map((e) => originalMirror.getField(e.simpleName).reflectee)
         .where((e) => e != null)
         .toList();
   }
 
   Map toMap() => {
     'name': Indexable.getDocgenObject(mirror, owningLibrary).docName,
     'parameters': parameters
   };
 }