Update dart_style to 1.2.8.

It's a small change but does include some formatting differences, so
should be coordinated with rolling the pre-built SDK in case those
differences affect code in the SDK.

Change-Id: I6d7e049547f8b4d7e3a14c2424db6286ad76ec35
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/105360
Reviewed-by: Jonas Termansen <sortie@google.com>
Commit-Queue: Jonas Termansen <sortie@google.com>

DEPS

@@ -79,7 +79,7 @@ vars = {
   #     and land the review.
   #
   # For more details, see https://github.com/dart-lang/sdk/issues/30164
-  "dart_style_tag": "1.2.7",  # Please see the note above before updating.
+  "dart_style_tag": "1.2.8",  # Please see the note above before updating.
 
   "dartdoc_tag" : "v0.28.2",
   "fixnum_tag": "0.10.9",