Add logging system to pub and sprinkle some logging in.

utils/pub/command_install.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_install;
 
 import 'entrypoint.dart';
+import 'log.dart' as log;
 import 'pub.dart';
 
 /** Handles the `install` pub command. */
 class InstallCommand extends PubCommand {
   String get description => "install the current package's dependencies";
   String get usage => "pub install";
 
   Future onRun() {
     return entrypoint.installDependencies().transform((_) {
-      print("Dependencies installed!");
+      log.message("Dependencies installed!");
     });
   }
 }