Roll back yaml version in DEPS to 2.1.11

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 102 matching lines @@
   "test_tag": "@0.12.15+6",
   "typed_data_tag": "@1.1.3",
   "usage_tag": "@v2.2.2",
   "utf_tag": "@0.9.0+3",
   "watcher_tag": "@0.9.7+3",
   "web_components_rev": "@6349e09f9118dce7ae1b309af5763745e25a9d61",
   "web_socket_channel_tag": "@1.0.4",
   "WebCore_rev": "@a86fe28efadcfc781f836037a80f27e22a5dad17",
   "when_tag": "@0.2.0+2",
   "which_tag": "@0.1.3+1",
-  "yaml_tag": "@2.1.12",
+  "yaml_tag": "@2.1.11",
   "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":
@@ skipping 458 matching lines @@
     # 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"],
   },
 ]