Revert "Remove "Windows toolchain" step from DEPS. May only be used by gyp."

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.
 
 # IMPORTANT:
 # Before adding or updating dependencies, please review the documentation here:
 # https://github.com/dart-lang/sdk/wiki/Adding-and-Updating-Dependencies
 
 vars = {
   # The dart_root is the root of our sdk checkout. This is normally
@@ skipping 15 matching lines @@
   "chromium_git": "https://chromium.googlesource.com",
   "fuchsia_git": "https://fuchsia.googlesource.com",
 
   # IMPORTANT:
   # This should only be used for local testing. Before adding a new package,
   # request a mirror of the package you need. To request a mirror, file an issue
   # on github and add the label 'area-infrastructure'.
   # "github_dartlang": "https://github.com/dart-lang/%s.git",
 
   "co19_rev": "@dec2b67aaab3bb7339b9764049707e71e601da3d",
+  "gyp_rev": "@4801a5331ae62da9769a327f11c4213d32fb0dad",
 
   # Revisions of GN related dependencies. This should match the revision
   # pulled by Flutter.
   "buildtools_revision": "@057ef89874e3c622248cf99259434fdc683c4e30",
 
   # Scripts that make 'git cl format' work.
   "clang_format_scripts_rev": "@c09c8deeac31f05bd801995c475e7c8070f9ecda",
 
   "gperftools_revision": "@02eeed29df112728564a5dde6417fa4622b57a06",
 
@@ skipping 86 matching lines @@
   "usage_tag": "@3.3.0",
   "utf_tag": "@0.9.0+3",
   "watcher_tag": "@0.9.7+3",
   "web_socket_channel_tag": "@1.0.4",
   "WebCore_rev": "@3c45690813c112373757bbef53de1602a62af609",
   "yaml_tag": "@2.1.12",
   "zlib_rev": "@c3d0a6190f2f8c924a05ab6cc97b8f975bddd33f",
 }
 
 deps = {
+  # Stuff needed for GYP to run.
+  Var("dart_root") + "/third_party/gyp":
+      Var('chromium_git') + '/external/gyp.git' + Var("gyp_rev"),
+
   # Stuff needed for GN build.
   Var("dart_root") + "/buildtools":
      Var("fuchsia_git") + "/buildtools" + Var("buildtools_revision"),
   Var("dart_root") + "/buildtools/clang_format/script":
     Var("chromium_git") + "/chromium/llvm-project/cfe/tools/clang-format.git" +
     Var("clang_format_scripts_rev"),
 
   Var("dart_root") + "/tests/co19/src":
       Var("github_mirror") + "co19.git" + Var("co19_rev"),
 
@@ skipping 313 matching lines @@
     'name': 'download_android_tools',
     'pattern': '.',
     'action': ['python', 'sdk/tools/android/download_android_tools.py'],
   },
   {
     'name': 'buildtools',
     'pattern': '.',
     'action': ['python', 'sdk/tools/buildtools/update.py'],
   },
   {
+    # 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/generate_buildfiles.py"],
   },
 ]