[infra] Upgrade checked-in SDKs to 2.18.0-271.7.beta

Change-Id: I8e7dc1d6241783d47c1e0b62fe039a7fef16ca44
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/256000
Reviewed-by: William Hesse <[email protected]>
Commit-Queue: Alexander Thomas <[email protected]>

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:2.18.0-271.0.dev",
+  "sdk_tag": "version:2.18.0-271.7.beta",
 
   # co19 is a cipd package. Use update.sh in tests/co19[_2] to update these
   # hashes.