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

pkg/docgen/lib/docgen.dart

Index: pkg/docgen/lib/docgen.dart
diff --git a/pkg/docgen/lib/docgen.dart b/pkg/docgen/lib/docgen.dart
index 9c8fc8dc11c47de099451aa232d2fa4c27ec267b..bb500908213638ded4d20475edf6d514986893cd 100644
--- a/pkg/docgen/lib/docgen.dart
+++ b/pkg/docgen/lib/docgen.dart
@@ -661,6 +661,11 @@ class _Viewer {
   /// 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',