Add logging system to pub and sprinkle some logging in.

utils/pub/system_cache.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 system_cache;
 
 import 'dart:io';
 
 import 'git_source.dart';
 import 'hosted_source.dart';
 import 'io.dart';
 import 'io.dart' as io show createTempDir;
+import 'log.dart' as log;
 import 'package.dart';
 import 'sdk_source.dart';
 import 'source.dart';
 import 'source_registry.dart';
 import 'utils.dart';
 import 'version.dart';
 
 /**
  * The system-wide cache of installed packages.
  *
@@ skipping 72 matching lines @@
   /// temp directory to ensure that it's on the same volume as the pub system
   /// cache so that it can move the directory from it.
   Future<Directory> createTempDir() {
     return ensureDir(tempDir).chain((temp) {
       return io.createTempDir(join(temp, 'dir'));
     });
   }
 
   /// Delete's the system cache's internal temp directory.
   Future deleteTempDir() {
+    log.fine('Clean up system cache temp directory $tempDir.');
     return dirExists(tempDir).chain((exists) {
       if (!exists) return new Future.immediate(null);
       return deleteDir(tempDir);
     });
   }
 }