Ensure that lists are reified.

lib/src/parser.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'arg_parser.dart';
 import 'arg_results.dart';
 import 'option.dart';
 
 final _SOLO_OPT = new RegExp(r'^-([a-zA-Z0-9])$');
 final _ABBR_OPT = new RegExp(r'^-([a-zA-Z0-9]+)(.*)$');
@@ skipping 240 matching lines @@
   /// a flag.
   void setOption(Map results, Option option, String value) {
     assert(!option.isFlag);
 
     if (!option.isMultiple) {
       _validateAllowed(option, value);
       results[option.name] = value;
       return;
     }
 
-    var list = results.putIfAbsent(option.name, () => []);
+    var list = results.putIfAbsent(option.name, () => <String>[]);
 
     if (option.splitCommas) {
       for (var element in value.split(",")) {
         _validateAllowed(option, element);
         list.add(element);
       }
     } else {
       _validateAllowed(option, value);
       list.add(value);
     }
   }
 
   /// Validates and stores [value] as the value for [option], which must be a
   /// flag.
   void setFlag(Map results, Option option, bool value) {
     assert(option.isFlag);
     results[option.name] = value;
   }
 
   /// Validates that [value] is allowed as a value of [option].
   void _validateAllowed(Option option, String value) {
     if (option.allowed == null) return;
 
     validate(option.allowed.contains(value),
         '"$value" is not an allowed value for option "${option.name}".');
   }
 }