GN: Build app snapshots for the SDK tools.

utils/invoke_dart.gni

Index: utils/invoke_dart.gni
diff --git a/utils/invoke_dart.gni b/utils/invoke_dart.gni
index 66705f712b2e50a6f55a8f0a435560dc8126713a..47adbff3f1dcfc95a934cadabd8a31768568b23f 100644
--- a/utils/invoke_dart.gni
+++ b/utils/invoke_dart.gni
@@ -2,6 +2,10 @@
 # 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.
 
+declare_args() {
+  dart_app_snapshot_kind = "app-after-run"
+}
+
 _dart_root = rebase_path("..")
 
 template("invoke_dart") {
@@ -50,7 +54,9 @@ template("invoke_dart") {
 
 template("application_snapshot") {
   assert(defined(invoker.main_dart), "Must specify 'main_dart'")
+  assert(defined(invoker.training_args), "Must specify 'training_args'")
   main_dart = invoker.main_dart
+  training_args = invoker.training_args
   name = target_name
   if (defined(invoker.name)) {
     name = invoker.name
@@ -85,7 +91,8 @@ template("application_snapshot") {
     args = [
       "--packages=$dot_packages",
       "--snapshot=$abs_output",
-      main_file
-    ]
+      "--snapshot-kind=$dart_app_snapshot_kind",
+      main_file,
+    ] + training_args
   }
 }