Fixed implementation of ArgResults.options to really use Iterable<String> instead of Iterable<dynam…

lib/src/arg_results.dart

 // Copyright (c) 2014, 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.
 
 library args.src.arg_results;
 
 import 'package:collection/wrappers.dart';
 
 import 'arg_parser.dart';
 
@@ skipping 52 matching lines @@
     }
 
     return _parser.options[name].getOrDefault(_parsed[name]);
   }
 
   /// Get the names of the available options as an [Iterable].
   ///
   /// This includes the options whose values were parsed or that have defaults.
   /// Options that weren't present and have no default will be omitted.
   Iterable<String> get options {
-    var result = new Set.from(_parsed.keys);
+    var result = new Set<String>.from(_parsed.keys);
 
     // Include the options that have defaults.
     _parser.options.forEach((name, option) {
       if (option.defaultValue != null) result.add(name);
     });
 
     return result;
   }
 
   /// Returns `true` if the option with [name] was parsed from an actual
   /// argument.
   ///
   /// Returns `false` if it wasn't provided and the default value or no default
   /// value would be used instead.
   bool wasParsed(String name) {
     var option = _parser.options[name];
     if (option == null) {
       throw new ArgumentError('Could not find an option named "$name".');
     }
 
     return _parsed.containsKey(name);
   }
 }