updated logging package to accept functions or other non-string objects when logging

pkg/logging/README.md

 ## Initializing
 
 By default, the logging package does not do anything useful with the
 log messages. You must configure the logging level and add a handler
 for the log messages.
 
 Here is a simple logging configuration that logs all messages
 via `print`.
 
 ```dart
@@ skipping 21 matching lines @@
 
 Here is an example of logging a debug message and an error:
 
 ```dart
 var future = doSomethingAsync().then((result) {
   log.fine('Got the result: $result');
   processResult(result);
 }).catchError((e, stackTrace) => log.severe('Oh noes!', e, stackTrace));
 ```
 
+When logging more complex messages, you can pass a closure instead
+that will be evaluated only if the message is actually logged:
+
+```dart
+  log.fine(() => [1, 2, 3, 4, 5].map((e) => e * 4).join("-"));
+```
+
 See the [Logger] class for the different logging methods.