Fix the built-in help command in args.

pkg/args/lib/src/help_command.dart

Index: pkg/args/lib/src/help_command.dart
diff --git a/pkg/args/lib/src/help_command.dart b/pkg/args/lib/src/help_command.dart
index 283f3074fe9f75bd2c64f0b4e97833dfb8a24631..f477b4740e630160700eb34eaee325a9af791726 100644
--- a/pkg/args/lib/src/help_command.dart
+++ b/pkg/args/lib/src/help_command.dart
@@ -13,11 +13,11 @@ class HelpCommand extends Command {
   final name = "help";
   String get description =>
       "Display help information for ${runner.executableName}.";
-  String get usage => "${runner.executableName} help [command]";
+  String get invocation => "${runner.executableName} help [command]";
 
   void run() {
     // Show the default help if no command was specified.
-    if (options.rest.isEmpty) {
+    if (argResults.rest.isEmpty) {
       runner.printUsage();
       return;
     }
@@ -28,7 +28,7 @@ class HelpCommand extends Command {
     var command = null;
     var commandString = runner.executableName;
 
-    for (var name in options.rest) {
+    for (var name in argResults.rest) {
       if (commands.isEmpty) {
         command.usageException(
             'Command "$commandString" does not expect a subcommand.');