pkg/logging: use UnmodifiableMapView from dart:collection

pkg/logging/README.md

Index: pkg/logging/README.md
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+## 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.