Change build.py to be like ninja.py and only use a single GN/Ninja invocation to cross-build the SD…

utils/application_snapshot.gni

Index: utils/application_snapshot.gni
diff --git a/utils/application_snapshot.gni b/utils/application_snapshot.gni
index 9a317e291ecc31e597738f5755d062de978caa7a..03661fb49818efe7efc5850c4413abb2ca96a658 100644
--- a/utils/application_snapshot.gni
+++ b/utils/application_snapshot.gni
@@ -6,6 +6,12 @@ import("../build/compiled_action.gni")
 
 _dart_root = get_path_info("..", "abspath")
 
+declare_args() {
+  # Default to building app-jit snapshots. The simulator and cross builds
+  # override this to script snapshots to cut down on build time.
+  dart_snapshot_kind = "app-jit"
+}
+
 template("application_snapshot") {
   assert(defined(invoker.main_dart), "Must specify 'main_dart'")
   assert(defined(invoker.training_args), "Must specify 'training_args'")
@@ -41,11 +47,18 @@ template("application_snapshot") {
     abs_output = rebase_path(output)
     main_file = rebase_path(main_dart)
 
-    args = [
-             "--packages=$dot_packages",
-             "--snapshot=$abs_output",
-             "--snapshot-kind=app-jit",
-             main_file,
-           ] + training_args
+    args = [ "--packages=$dot_packages",
+             "--snapshot=$abs_output" ]
+
+    if (dart_snapshot_kind == "script") {
+      args += ["--snapshot-kind=script",
+               main_file ]
+      assert(training_args != "", "Ignoring unused argument")
+    } else if (dart_snapshot_kind == "app-jit") {
+      args += ["--snapshot-kind=app-jit",
+               main_file ] + training_args
+    } else {
+      assert(false, "Bad dart_snapshot_kind: $dart_snapshot_kind")
+    }
   }
 }