Fix apidoc build step on Windows.

utils/apidoc/apidoc.gyp

Index: utils/apidoc/apidoc.gyp
diff --git a/utils/apidoc/apidoc.gyp b/utils/apidoc/apidoc.gyp
index 5ce55f83ab543ca565bd7755f5adf4a5cefb89c9..aafad1e1c8ddcebdd46d06cc69a26241a5651521 100644
--- a/utils/apidoc/apidoc.gyp
+++ b/utils/apidoc/apidoc.gyp
@@ -3,6 +3,11 @@
 # 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',
@@ -11,6 +16,11 @@
         '../../frog/dart-frog.gyp:frog',
         '../../runtime/dart-runtime.gyp:dart',
       ],
+      'conditions': [
+        ['OS=="win"', {
+          'msvs_cygwin_dirs': ['<(cygwin_dir)'],
+        }],
+      ],
       'actions': [
         {
           'action_name': 'run_apidoc',