Add dart2js.bat for running dart2js on Windows.

dart/utils/compiler/compiler.gyp

 # Copyright (c) 2012, 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.
 
 {
   'variables': {
     'dart_dir': '../..',
   },
   'targets': [
     {
       'target_name': 'dart2js',
       'type': 'none',
       'dependencies': [
         '../../runtime/dart-runtime.gyp:dart',
       ],
       'actions': [
         {
           'action_name': 'build_dart2js',
           'inputs': [
             '<(PRODUCT_DIR)/<(EXECUTABLE_PREFIX)dart<(EXECUTABLE_SUFFIX)',
             'build_helper.dart',
           ],
           'outputs': [
             '<(PRODUCT_DIR)/dart2js',
+            '<(PRODUCT_DIR)/dart2js.bat',
             '<(PRODUCT_DIR)/dart2js_developer',
+            '<(PRODUCT_DIR)/dart2js_developer.bat',
           ],
           'action': [
             '<(PRODUCT_DIR)/<(EXECUTABLE_PREFIX)dart<(EXECUTABLE_SUFFIX)',
             '--enable-checked-mode',
             'build_helper.dart',
             # Note: it would seem more straight-forward to pass in
             # '<(PRODUCT_DIR)/<(EXECUTABLE_PREFIX)dart<(EXECUTABLE_SUFFIX)'.
             # Unfortunately, there is some strange interaction with
             # GYP so it doesn't work.
             '<(PRODUCT_DIR)',
             '<(EXECUTABLE_PREFIX)dart<(EXECUTABLE_SUFFIX)',
             'dart2js',
             'dart2js_developer',
             '<(dart_dir)',
           ],
         },
       ],
     },
   ],
 }