Fix the built-in help command in args.

pkg/args/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.
 
 library utils;
 
 import 'dart:async';
 
 import 'package:args/args.dart';
 import 'package:args/command_runner.dart';
@@ skipping 57 matching lines @@
 void throwsIllegalArg(function, {String reason: null}) {
   expect(function, throwsArgumentError, reason: reason);
 }
 
 void throwsFormat(ArgParser parser, List<String> args) {
   expect(() => parser.parse(args), throwsFormatException);
 }
 
 Matcher throwsUsageError(message, usage) {
   return throwsA(predicate((error) {
-    expect(error, new isInstanceOf<UsageError>());
+    expect(error, new isInstanceOf<UsageException>());
     expect(error.message, message);
     expect(error.usage, usage);
     return true;
   }));
 }