Fix apidoc build step on Windows.

utils/apidoc/apidoc.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': {
+    # These variables are used in the creation of the .vcproj file on
+    # Windows.
+    'cygwin_dir': '../../third_party/cygwin',
+  },
   'targets': [
     {
       'target_name': 'api_docs',
       'type': 'none',
       'dependencies': [
         '../../frog/dart-frog.gyp:frog',
         '../../runtime/dart-runtime.gyp:dart',
       ],
+      'conditions': [
+        ['OS=="win"', {
+          'msvs_cygwin_dirs': ['<(cygwin_dir)'],
+        }],
+      ],
       'actions': [
         {
           'action_name': 'run_apidoc',
           'inputs': [
             '<(PRODUCT_DIR)/dart',
             '<!@(["python", "scripts/list_files.py"])',
           ],
           'outputs': [
             '<(PRODUCT_DIR)/api_docs',
           ],
           'action': [
             '<(PRODUCT_DIR)/dart',
             'apidoc.dart',
             '--out=<(PRODUCT_DIR)/api_docs',
             '--mode=live-nav'
           ],
         },
       ],
     }
   ],
 }