pkg/args: expose immutable collections

pkg/args/lib/src/options.dart

 library options;
 
+import 'package:unmodifiable_collection/unmodifiable_collection.dart';
+
 /**
  * A command-line option. Includes both flags and options which take a value.
  */
 class Option {
   final String name;
   final String abbreviation;
   final List<String> allowed;
   final defaultValue;
   final Function callback;
   final String help;
   final Map<String, String> allowedHelp;
   final bool isFlag;
   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.');
     } else if (name.startsWith('-')) {
       throw new ArgumentError('Name $name cannot start with "-".');
     }
 
     // Ensure name does not contain any invalid characters.
     if (_invalidChars.hasMatch(name)) {
       throw new ArgumentError('Name "$name" contains invalid characters.');
     }
 
     if (abbreviation != null) {
       if (abbreviation.length != 1) {
         throw new ArgumentError('Abbreviation must be null or have length 1.');
       } else if(abbreviation == '-') {
         throw new ArgumentError('Abbreviation cannot be "-".');
       }
 
       if (_invalidChars.hasMatch(abbreviation)) {
         throw new ArgumentError('Abbreviation is an invalid character.');
       }
     }
   }
 
   static final _invalidChars = new RegExp(r'''[ \t\r\n"'\\/]''');
 }