Add dartdev as a pkg/ utility that should have a snapshot generated and added in the Dart SDK

The formatting churn is from being prompted with the required `git cl format` command

Change-Id: Ic2582f720e30f5d9a35fad05709455dad1068ad6
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/136842
Commit-Queue: Jaime Wren <jwren@google.com>
Reviewed-by: Alexander Thomas <athom@google.com>

utils/dartdev/BUILD.gn

@@ -0,0 +1,10 @@
+# Copyright (c) 2020, the Dart project authors.  Please see the AUTHORS file
+# for details. All rights reserved. Use of this source code is governed by a
+# BSD-style license that can be found in the LICENSE file.
+
+import("../application_snapshot.gni")
+
+application_snapshot("dartdev") {
+  main_dart = "../../pkg/dartdev/bin/dartdev.dart"
+  training_args = [ "--help" ]
+}