Change dartdocgen --serve to use pub get rather than pub upgrade

pkg/docgen/lib/src/viewer.dart

 // Copyright (c) 2014, 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.
 
 /// Convenience methods wrapped up in a class to pull down the docgen viewer for
 /// a viewable website, and start up a server for viewing.
 library docgen.viewer;
 
 import 'dart:io';
 
@@ skipping 71 matching lines @@
   // If the viewer code is already there, then don't clone again.
   if (_dartdocViewerDir.existsSync()) {
     _moveDirectoryAndServe();
   } else {
     var processResult = Process.runSync('git', ['clone', '-b', 'master',
         'https://github.com/dart-lang/dartdoc-viewer.git'], runInShell: true);
 
     if (processResult.exitCode == 0) {
       /// Move the generated json/yaml docs directory to the dartdoc-viewer
       /// directory, to run as a webpage.
-      var processResult = Process.runSync(gen.pubScript, ['upgrade'],
+      var processResult = Process.runSync(gen.pubScript, ['get'],
           runInShell: true, workingDirectory: _viewerCodePath);
       print('process output: ${processResult.stdout}');
       print('process stderr: ${processResult.stderr}');
 
       var dir = new Directory(gen.outputDirectory == null ? 'docs' :
           gen.outputDirectory);
       _webDocsDir = new Directory(path.join(_viewerCodePath, 'web', 'docs'));
       if (dir.existsSync()) {
         // Move the docs folder to dartdoc-viewer/client/web/docs
         dir.renameSync(_webDocsDir.path);
@@ skipping 89 matching lines @@
         // Serve up file contents.
         file.openRead().pipe(response).catchError((e) {
           print('HttpServer: error while closing the response stream $e');
         });
       }
     }, onError: (e) {
       print('HttpServer: an error occured $e');
     });
   });
 }