Enables GN build on windows

DEPS

 # 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.
 
 vars = {
   # The dart_root is the root of our sdk checkout. This is normally
   # simply sdk, but if using special gclient specs it can be different.
   "dart_root": "sdk",
 
   # We use mirrors of all github repos to guarantee reproducibility and
@@ skipping 26 matching lines @@
 
   # Revisions of /third_party/* dependencies.
   "args_tag": "@0.13.5",
   "async_tag": "@1.11.2",
   "barback-0.13.0_rev": "@34853",
   "barback-0.14.0_rev": "@36398",
   "barback-0.14.1_rev": "@38525",
   "barback_tag" : "@0.15.2+9",
   "bazel_worker_tag": "@0.1.1",
   "boolean_selector_tag" : "@1.0.2",
-  "boringssl_gen_rev": "@922830c0aad900dd3d143eef1ba06faa83fe263b",
+  "boringssl_gen_rev": "@62c20247d582444cb2804f9ea4e3abaa6e47f6a5",
   "boringssl_rev" : "@8d343b44bbab829d1a28fdef650ca95f7db4412e",
   "charcode_tag": "@1.1.0",
   "chrome_rev" : "@19997",
   "cli_util_tag" : "@0.0.1+3",
   "code_transformers_tag": "@v0.4.2+3",
   "collection_tag": "@1.9.1",
   "convert_tag": "@2.0.1",
   "crypto_tag" : "@2.0.1",
   "csslib_tag" : "@0.13.2",
   "dart2js_info_tag" : "@0.3.0",
@@ skipping 497 matching lines @@
       Var('dart_root') + "/third_party/clang.tar.gz.sha1",
     ],
   },
   {
     # Pull clang if needed or requested via GYP_DEFINES.
     'name': 'gn_clang',
     'pattern': '.',
     'action': ['python', 'sdk/tools/clang/scripts/update.py', '--if-needed'],
   },
   {
+    # Update the Windows toolchain if necessary.
+    'name': 'win_toolchain',
+    'pattern': '.',
+    'action': ['python', 'sdk/build/vs_toolchain.py', 'update'],
+  },
+  {
     "pattern": ".",
     "action": ["python", Var("dart_root") + "/tools/gyp_dart.py"],
   },
 ]