Add logging system to pub and sprinkle some logging in.

utils/pub/system_cache.dart

Index: utils/pub/system_cache.dart
diff --git a/utils/pub/system_cache.dart b/utils/pub/system_cache.dart
index 0e4822c6c37f00eda1a31c65ff7a59cc5e36d4ee..73df7f2f65076ecef127d286c4f88b0fd03a938f 100644
--- a/utils/pub/system_cache.dart
+++ b/utils/pub/system_cache.dart
@@ -8,6 +8,7 @@ import 'dart:io';
 
 import 'io.dart';
 import 'io.dart' as io show createTempDir;
+import 'log.dart' as log;
 import 'package.dart';
 import 'source.dart';
 import 'source_registry.dart';
@@ -89,6 +90,7 @@ class SystemCache {
 
   /// 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);