Update to the latest package:async

Change-Id: Ie097d1c49ac0f27f0d9afbc8f8fa01e1bd4f07c8
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/213106
Auto-Submit: Nate Bosch <nbosch@google.com>
Commit-Queue: Nate Bosch <nbosch@google.com>
Commit-Queue: Jake Macdonald <jakemac@google.com>
Reviewed-by: Jake Macdonald <jakemac@google.com>

.dart_tool/package_config.json

@@ -11,7 +11,7 @@
     "constraint, update this by running tools/generate_package_config.dart."
   ],
   "configVersion": 2,
-  "generated": "2021-09-10T10:06:41.987732",
+  "generated": "2021-09-10T11:44:31.694550",
   "generator": "tools/generate_package_config.dart",
   "packages": [
     {
@@ -112,7 +112,7 @@
       "name": "async",
       "rootUri": "../third_party/pkg/async",
       "packageUri": "lib/",
-      "languageVersion": "2.12"
+      "languageVersion": "2.14"
     },
     {
       "name": "async_helper",

DEPS

@@ -73,7 +73,7 @@ vars = {
 
   # Revisions of /third_party/* dependencies.
   "args_rev": "3b3f55766af13d895d2020ec001a28e8dc147f91",
-  "async_rev": "ecb3835c5b746f7762327d5f57bdc92e0a2e0450",
+  "async_rev": "c64220396e0fa2f7b380590abfedbd25635cd7a0",
   "bazel_worker_rev": "0885637b037979afbf5bcd05fd748b309fd669c0",
   "benchmark_harness_rev": "c546dbd9f639f75cd2f75de8df2eb9f8ea15e8e7",
   "boolean_selector_rev": "665e6921ab246569420376f827bff4585dff0b14",