Update async package to include fix for Stream changes

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 24 matching lines @@
   # pulled by Flutter.
   "buildtools_revision": "@057ef89874e3c622248cf99259434fdc683c4e30",
 
   # Scripts that make 'git cl format' work.
   "clang_format_scripts_rev": "@c09c8deeac31f05bd801995c475e7c8070f9ecda",
 
   "gperftools_revision": "@02eeed29df112728564a5dde6417fa4622b57a06",
 
   # Revisions of /third_party/* dependencies.
   "args_tag": "@0.13.7",
-  "async_tag": "@1.13.0",
+  "async_tag": "@daf66909019d2aaec1721fc39d94ea648a9fdc1d",
   "barback-0.13.0_rev": "@34853",
   "barback-0.14.0_rev": "@36398",
   "barback-0.14.1_rev": "@38525",
   "barback_tag" : "@0.15.2+11",
   "bazel_worker_tag": "@v0.1.4",
   "boolean_selector_tag" : "@1.0.2",
   "boringssl_gen_rev": "@753224969dbe43dad29343146529727b5066c0f3",
   "boringssl_rev" : "@d519bf6be0b447fb80fbc539d4bff4479b5482a2",
   "charcode_tag": "@v1.1.1",
   "chrome_rev" : "@19997",
@@ skipping 436 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"],
   },
 ]