Add logging system to pub and sprinkle some logging in.

utils/pub/command_update.dart

 // Copyright (c) 2012, 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.
 
 library command_update;
 
 import 'entrypoint.dart';
+import 'log.dart' as log;
 import 'pub.dart';
 
 /** Handles the `update` pub command. */
 class UpdateCommand extends PubCommand {
   String get description =>
     "update the current package's dependencies to the latest versions";
 
   String get usage => 'pub update [dependencies...]';
 
   Future onRun() {
     var future;
     if (commandOptions.rest.isEmpty) {
       future = entrypoint.updateAllDependencies();
     } else {
       future = entrypoint.updateDependencies(commandOptions.rest);
     }
-    return future.transform((_) => print("Dependencies updated!"));
+    return future.transform((_) => log.message("Dependencies updated!"));
   }
 }