Improve Logger.root (static final instead of getter that looks the cached instance up through the factory constructor)

(note: static fields are lazy anyway)
diff --git a/lib/logging.dart b/lib/logging.dart
index f108ed3..c8417fc 100644
--- a/lib/logging.dart
+++ b/lib/logging.dart
@@ -229,7 +229,7 @@
   }
 
   /** Top-level root [Logger]. */
-  static Logger get root => new Logger('');
+  static final Logger root = new Logger('');
 
   /** All [Logger]s in the system. */
   static final Map<String, Logger> _loggers = <String, Logger>{};