[infra] Update checked-in SDKs to 2.9.0-15.0.dev

dart-lang/sdk#42274

Change-Id: I4ea2c533e4f47d42313a500c7c04c60c8a5c714b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/150931
Auto-Submit: Alexander Thomas <athom@google.com>
Commit-Queue: Alexander Thomas <athom@google.com>
Commit-Queue: William Hesse <whesse@google.com>
Reviewed-by: William Hesse <whesse@google.com>

Author: athomas

DEPS

@@ -39,7 +39,7 @@ vars = {
 
   # Checked-in SDK version. The checked-in SDK is a Dart SDK distribution in a
   # cipd package used to run Dart scripts in the build and test infrastructure.
-  "sdk_tag": "version:2.9.0-10.0.dev",
+  "sdk_tag": "version:2.9.0-15.0.dev",
 
   # co19 is a cipd package. Use update.sh in tests/co19[_2] to update these
   # hashes. It requires access to the dart-build-access group, which EngProd