Add support for hidden options to pkg/args.

pkg/args/lib/src/options.dart

Index: pkg/args/lib/src/options.dart
diff --git a/pkg/args/lib/src/options.dart b/pkg/args/lib/src/options.dart
index 5f1f48405ce04c627881789875c71c731cd641ff..06eeaf515ee7fe3bd15cbb221b93405d70124749 100644
--- a/pkg/args/lib/src/options.dart
+++ b/pkg/args/lib/src/options.dart
@@ -16,10 +16,12 @@ class Option {
   final bool isFlag;
   final bool negatable;
   final bool allowMultiple;
+  final bool isHidden;
 
   Option(this.name, this.abbreviation, this.help, List<String> allowed,
       Map<String, String> allowedHelp, this.defaultValue, this.callback,
-      {this.isFlag, this.negatable, this.allowMultiple: false}) :
+      {this.isFlag, this.negatable, this.allowMultiple: false,
+      this.isHidden: false}) :
         this.allowed = allowed == null ?
             null : new UnmodifiableListView(allowed),
         this.allowedHelp = allowedHelp == null ?