Update to latest package:async

Change-Id: Id072725c6e7c663c0fabdca5d2f001507fe6d9dc
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/168775
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>

Author: natebosch

DEPS

@@ -66,7 +66,7 @@ vars = {
 
   # Revisions of /third_party/* dependencies.
   "args_tag": "1.6.0",
-  "async_rev": "38ace5fa83697928d5f29911e346e6311dd00857",
+  "async_rev": "6d2de25f208b011d79d056115256d3840229723b",
   "bazel_worker_rev": "26680d5e249b249c7216ab2fed0ac8ed4ee285c5",
   "benchmark_harness_rev": "ec6b646f5443faa871e126ac1ba248c94ca06257",
   "boolean_selector_rev": "665e6921ab246569420376f827bff4585dff0b14",