Fix the --version option.

lib/src/options.dart

 // Copyright (c) 2014, 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.
 
 /// Set of flags and options passed to the compiler
 library dev_compiler.src.options;
 
 import 'dart:io';
 
 import 'package:args/args.dart';
@@ skipping 270 matching lines @@
       help: 'Additional resources to serve', defaultsTo: '')
   ..addFlag('source-maps',
       help: 'Whether to emit source map files', defaultsTo: true)
   ..addOption('runtime-dir',
       help: 'Where to find dev_compiler\'s runtime files', defaultsTo: null)
   ..addFlag('arrow-fn-bind-this',
       help: 'Work around `this` binding in => functions')
 
   // general options
   ..addFlag('help', abbr: 'h', help: 'Display this message')
-  ..addFlag('version', help: 'Display the Dev Compiler verion')
+  ..addFlag('version',
+      negatable: false, help: 'Display the Dev Compiler verion')
   ..addFlag('server', help: 'Run as a development server.', defaultsTo: false)
   ..addFlag('hashing',
       help: 'Enable hash-based file caching.', defaultsTo: null)
   ..addFlag('widget',
       help: 'Serve embedded widget with static errors and warnings.',
       defaultsTo: true)
   ..addOption('host',
       help: 'Host name or address to serve files from, e.g. --host=0.0.0.0\n'
           'to listen on all interfaces (used only when --serve is on)',
       defaultsTo: 'localhost')
@@ skipping 65 matching lines @@
     // The pub-cache directory is two levels up, but we verify that the layout
     // looks correct.
     if (path.basename(dir) != 'dev_compiler') return null;
     dir = path.dirname(dir);
     if (path.basename(dir) != 'global_packages') return null;
     dir = path.dirname(dir);
     return path.join(dir, cacheDir, 'lib', 'runtime');
   }
   return null;
 }