Combine unmodifiable_collection package into collection_helpers.

pkg/args/lib/src/options.dart

 library options;
 
-import 'package:unmodifiable_collection/unmodifiable_collection.dart';
+import 'package:collection_helpers/wrappers.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;
@@ skipping 32 matching lines @@
       }
 
       if (_invalidChars.hasMatch(abbreviation)) {
         throw new ArgumentError('Abbreviation is an invalid character.');
       }
     }
   }
 
   static final _invalidChars = new RegExp(r'''[ \t\r\n"'\\/]''');
 }