pkg/logging: use UnmodifiableMapView from dart:collection

pkg/logging/README.md

+Support for logging.

[sethladd, 2014/07/24 16:48:13]

Nit: start line with # ?

[kevmoo, 2014/07/24 17:44:18]

Done.

+

[sethladd, 2014/07/24 16:48:13]

READMEs often include notes about where to file bugs and the LICENSE

[kevmoo, 2014/07/24 17:44:18]

There's already a license file in the package. We don't have "contributing" docs
in most other readmes (including where to file bugs). Perhaps we should go the
way of github and have the convention of using CONTRIBUTING.md in a package to
include details about filing issues and patches

+## 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
+Logger.root.level = Level.ALL;
+Logger.root.onRecord.listen((LogRecord rec) {
+  print('${rec.level.name}: ${rec.time}: ${rec.message}');
+});
+```
+
+First, set the root [Level]. All messages at or above the level are
+sent to the [onRecord] stream.
+
+Then, listen on the [onRecord] stream for [LogRecord] events. The
+[LogRecord] class has various properties for the message, error,
+logger name, and more.
+
+## Logging messages
+
+Create a [Logger] with a unique name to easily identify the source
+of the log messages.
+
+```dart
+final Logger log = new Logger('MyClassName');
+```
+
+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));
+```
+
+See the [Logger] class for the different logging methods.