Make it strong mode clean.

test/utils.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 'dart:async';
 
 import 'package:args/args.dart';
 import 'package:args/command_runner.dart';
 import 'package:test/test.dart';
 
 class CommandRunnerWithFooter extends CommandRunner {
-  final usageFooter = "Also, footer!";
+  String get usageFooter => "Also, footer!";
 
   CommandRunnerWithFooter(String executableName, String description)
       : super(executableName, description);
 }
 
 class FooCommand extends Command {
   var hasRun = false;
 
   final name = "foo";
   final description = "Set a value.";
@@ skipping 61 matching lines @@
 }
 
 Matcher throwsUsageError(message, usage) {
   return throwsA(predicate((error) {
     expect(error, new isInstanceOf<UsageException>());
     expect(error.message, message);
     expect(error.usage, usage);
     return true;
   }));
 }