Fix the built-in help command in args.

pkg/args/test/command_runner_test.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 command_runner_test;
 
 import 'package:args/args.dart';
 import 'package:args/command_runner.dart';
 import 'package:unittest/unittest.dart';
 
@@ skipping 160 matching lines @@
         expect(() => runner.run(["foo", "--help"]), prints("""
 Set a value.
 
 Usage: test foo [arguments]
 -h, --help    Print this usage information.
 
 Run "test help" to see global options.
 """));
       });
     });
+
+    group("with help command", () {
+      test("with no command prints the usage", () {
+        expect(() => runner.run(["help"]), prints("""
+A test command runner.
+
+$_DEFAULT_USAGE
+"""));
+      });
+
+      test("with a command prints the usage for that command", () {
+        var command = new FooCommand();
+        runner.addCommand(command);
+
+        expect(() => runner.run(["help", "foo"]), prints("""
+Set a value.
+
+Usage: test foo [arguments]
+-h, --help    Print this usage information.
+
+Run "test help" to see global options.
+"""));
+    });
+
+      test("prints its own usage", () {
+        expect(() => runner.run(["help", "help"]), prints("""
+Display help information for test.
+
+Usage: test help [command]
+-h, --help    Print this usage information.
+
+Run "test help" to see global options.
+"""));
+      });
+    });
   });
 
   group("with a footer", () {
     setUp(() {
       runner = new CommandRunnerWithFooter("test", "A test command runner.");
     });
 
     test("includes the footer in the usage string", () {
       expect(runner.usage, equals("""
 A test command runner.
@@ skipping 60 matching lines @@
 
       expect(runner.run(["foo", "bar"]),
           throwsUsageError('Command "foo" does not take any arguments.', """
 Usage: test foo [arguments]
 -h, --help    Print this usage information.
 
 Run "test help" to see global options."""));
     });
   });
 }