Follow getopt in parsing option-like values.

lib/src/parser.dart

 // Copyright (c) 2013, 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 args.src.parser;
 
 import 'arg_parser.dart';
 import 'arg_results.dart';
 import 'option.dart';
 
@@ skipping 85 matching lines @@
         grammar, results, commandName, commandResults, rest, arguments);
   }
 
   /// Pulls the value for [option] from the second argument in [args].
   ///
   /// Validates that there is a valid value there.
   void readNextArgAsValue(Option option) {
     // Take the option argument from the next command line arg.
     validate(args.length > 0, 'Missing argument for "${option.name}".');
 
-    // Make sure it isn't an option itself.
-    validate(!_ABBR_OPT.hasMatch(current) && !_LONG_OPT.hasMatch(current),
-        'Missing argument for "${option.name}".');
-
     setOption(results, option, current);
     args.removeAt(0);
   }
 
   /// Tries to parse the current argument as a "solo" option, which is a single
   /// hyphen followed by a single letter.
   ///
   /// We treat this differently than collapsed abbreviations (like "-abc") to
   /// handle the possible value that may follow it.
   bool parseSoloOption() {
@@ skipping 164 matching lines @@
   }
 
   /// Validates that [value] is allowed as a value of [option].
   void _validateAllowed(Option option, String value) {
     if (option.allowed == null) return;
 
     validate(option.allowed.contains(value),
         '"$value" is not an allowed value for option "${option.name}".');
   }
 }