pkg/args: expose immutable collections

pkg/args/lib/src/options.dart

Index: pkg/args/lib/src/options.dart
diff --git a/pkg/args/lib/src/options.dart b/pkg/args/lib/src/options.dart
index 00c0ad8dc22b2f190b418b6436846cc054a4c139..5f1f48405ce04c627881789875c71c731cd641ff 100644
--- a/pkg/args/lib/src/options.dart
+++ b/pkg/args/lib/src/options.dart
@@ -1,5 +1,7 @@
 library options;
 
+import 'package:unmodifiable_collection/unmodifiable_collection.dart';
+
 /**
  * A command-line option. Includes both flags and options which take a value.
  */
@@ -15,9 +17,13 @@ class Option {
   final bool negatable;
   final bool allowMultiple;
 
-  Option(this.name, this.abbreviation, this.help, this.allowed,
-      this.allowedHelp, this.defaultValue, this.callback, {this.isFlag,
-      this.negatable, this.allowMultiple: false}) {
+  Option(this.name, this.abbreviation, this.help, List<String> allowed,
+      Map<String, String> allowedHelp, this.defaultValue, this.callback,
+      {this.isFlag, this.negatable, this.allowMultiple: false}) :
+        this.allowed = allowed == null ?
+            null : new UnmodifiableListView(allowed),
+        this.allowedHelp = allowedHelp == null ?
+            null : new UnmodifiableMapView(allowedHelp) {
 
     if (name.isEmpty) {
       throw new ArgumentError('Name cannot be empty.');