Update tools sdk to 3.8.0-171.0.dev

This is in preparation for https://dart-review.googlesource.com/c/sdk/+/397164
which needs a 3.8 sdk, as it introduces a package for the tests/ directory
which needs language version 3.8

Change-Id: I41e49355204c805bea14ac22c8bcfbbb6f28b489
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/414080
Auto-Submit: Sigurd Meldgaard <[email protected]>
Reviewed-by: Alexander Thomas <[email protected]>
Commit-Queue: Sigurd Meldgaard <[email protected]>

Author: sigurdm

DEPS

@@ -50,7 +50,7 @@ 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, which is automatically built on the release commits.
-  "sdk_tag": "git_revision:a8bfb132c5f7b9555d13ea79eaf0eaa77825824d",
+  "sdk_tag": "git_revision:2d5dfe32cf2e6b3c3d6b396885502a5402b4fc72",
 
   # co19 is a cipd package automatically generated for each co19 commit.
   # Use tests/co19/update.sh to update this hash.