Make it strong mode clean.

test/parse_test.dart

 // Copyright (c) 2012, 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 'package:test/test.dart';
 import 'package:args/args.dart';
 import 'utils.dart';
 
 void main() {
   group('ArgParser.parse()', () {
@@ skipping 99 matching lines @@
       test('for absent flags are invoked with the default value', () {
         var a;
         var parser = new ArgParser();
         parser.addFlag('a', defaultsTo: false, callback: (value) => a = value);
 
         parser.parse([]);
         expect(a, isFalse);
       });
 
       test('are invoked even if the flag is not present', () {
-        var a = 'not called';
+        var a = true;
         var parser = new ArgParser();
         parser.addFlag('a', callback: (value) => a = value);
 
         parser.parse([]);
         expect(a, isFalse);
       });
 
       test('for present options are invoked with the value', () {
         var a;
         var parser = new ArgParser();
@@ skipping 365 matching lines @@
         var parser = new ArgParser();
         parser.addFlag('woof');
 
         var results = parser.parse(['--woof', 'stop', '--', 'arg']);
         expect(results['woof'], isTrue);
         expect(results.rest, equals(['stop', '--', 'arg']));
       });
     });
   });
 }