Teach gen_snapshot how to create script snapshots if given a core snapshot to create them against.

runtime/bin/BUILD.gn

 # Copyright (c) 2014, 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("gypi_contents.gni")
 import("../runtime_args.gni")
 
 # Generate a resources.cc file for the service isolate without Observatory.
 action("gen_resources_cc") {
   visibility = [ ":*" ]  # Only targets in this file can see this.
@@ skipping 464 matching lines @@
     gen_snapshot_stamp_file,
   ]
 
   gen_snapshot_dir =
       get_label_info("../bin:gen_snapshot($host_toolchain)", "root_out_dir")
 
   script = rebase_path("../tools/create_snapshot_bin.py")
   args = [
     "--executable",
     rebase_path("$gen_snapshot_dir/gen_snapshot"),
+    "--snapshot_kind",
+    "core",
     "--vm_output_bin",
     rebase_path(vm_isolate_snapshot, root_build_dir),
-    "--output_bin",
+    "--isolate_output_bin",
     rebase_path(isolate_snapshot, root_build_dir),
     "--target_os",
     current_os,
     "--timestamp_file",
     rebase_path(gen_snapshot_stamp_file, root_build_dir),
   ]
 }
 
 action("generate_snapshot_file") {
   deps = [
@@ skipping 355 matching lines @@
   ]
   if (is_linux || is_android) {
     cflags = [ "-fPIC" ]
   }
   if (is_win) {
     libs = [ "dart.lib" ]
     abs_root_out_dir = rebase_path(root_out_dir)
     ldflags = [ "/LIBPATH:$abs_root_out_dir" ]
   }
 }