Update dart_style to 1.3.0.

The main changes are support for extension methods and NNBD, but a few
other small formatting tweaks are included too.

Change-Id: I0be45438d6b56ae5854c4677e58623ec93acaa60
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/118900
Commit-Queue: Alexander Thomas <athom@google.com>
Reviewed-by: Alexander Thomas <athom@google.com>
diff --git a/DEPS b/DEPS
index f7a2b6e..7d7529e 100644
--- a/DEPS
+++ b/DEPS
@@ -82,7 +82,7 @@
   #     and land the review.
   #
   # For more details, see https://github.com/dart-lang/sdk/issues/30164
-  "dart_style_tag": "1.2.8",  # Please see the note above before updating.
+  "dart_style_tag": "1.3.0",  # Please see the note above before updating.
 
   "dartdoc_rev" : "6934accd88c29a73cae26d0c4def3323efc2119c",
   "fixnum_tag": "0.10.9",