Allow command ArgParsers to be overridden.

lib/command_runner.dart

Index: lib/command_runner.dart
diff --git a/lib/command_runner.dart b/lib/command_runner.dart
index 1e33965cc0b55cacdd59640ef324bf21a2c21653..21dfa32d842f4b349be0b42b2e093bf60063e95a 100644
--- a/lib/command_runner.dart
+++ b/lib/command_runner.dart
@@ -67,7 +67,8 @@ Run "$executableName help <command>" for more information about a command.''';
   /// Global options should be registered with this parser; they'll end up
   /// available via [Command.globalResults]. Commands should be registered with
   /// [addCommand] rather than directly on the parser.
-  final argParser = new ArgParser();
+  ArgParser get argParser => _argParser;
+  final _argParser = new ArgParser();
 
   CommandRunner(this.executableName, this.description) {
     argParser.addFlag('help',
@@ -249,7 +250,8 @@ abstract class Command {
   /// the constructor); they'll end up available via [argResults]. Subcommands
   /// should be registered with [addSubcommand] rather than directly on the
   /// parser.
-  final argParser = new ArgParser();
+  ArgParser get argParser => _argParser;
+  final _argParser = new ArgParser();
 
   /// Generates a string displaying usage information for this command.
   ///