Allow passing in an existing ArgParser for a command.

pkg/args/test/command_test.dart

Index: pkg/args/test/command_test.dart
diff --git a/pkg/args/test/command_test.dart b/pkg/args/test/command_test.dart
index 50794256f1caf8b9adb777bea5b144737f77a1e6..6115ab1c2d323bd935f5c5d5b9944346cd3ae87d 100644
--- a/pkg/args/test/command_test.dart
+++ b/pkg/args/test/command_test.dart
@@ -9,6 +9,21 @@ import 'package:args/args.dart';
 
 main() {
   group('ArgParser.addCommand()', () {
+    test('creates a new ArgParser if none is given', () {
+      var parser = new ArgParser();
+      var command = parser.addCommand('install');
+      expect(parser.commands['install'], equals(command));
+      expect(command is ArgParser, isTrue);
+    });
+
+    test('uses the command parser if given one', () {
+      var parser = new ArgParser();
+      var command = new ArgParser();
+      var result = parser.addCommand('install', command);
+      expect(parser.commands['install'], equals(command));
+      expect(result, equals(command));
+    });
+
     test('throws on a duplicate command name', () {
       var parser = new ArgParser();
       parser.addCommand('install');