gen_snapshot now generates two snapshot buffers, one for the vm isolate

mojo/dart/embedder/test/BUILD.gn

Index: mojo/dart/embedder/test/BUILD.gn
diff --git a/mojo/dart/embedder/test/BUILD.gn b/mojo/dart/embedder/test/BUILD.gn
index f3f8fe5c5e99747a69ead268db2173f6dcf479ac..47888e20f7dd7ef8696bde57410717a9733de0b4 100644
--- a/mojo/dart/embedder/test/BUILD.gn
+++ b/mojo/dart/embedder/test/BUILD.gn
@@ -18,9 +18,11 @@ action("generate_test_snapshot_bin") {
     "//mojo/public/dart/src/natives.dart",
     "//mojo/public/dart/src/timer_queue.dart",
   ]
-  output = "$target_gen_dir/test_snapshot_gen.bin"
+  test_vm_isolate_snapshot = "$target_gen_dir/test_vm_isolate_snapshot.bin"
+  test_isolate_snapshot = "$target_gen_dir/test_isolate_snapshot.bin"
   outputs = [
-    output,
+    test_vm_isolate_snapshot,
+    test_isolate_snapshot,
   ]
 
   gen_snapshot_dir =
@@ -36,8 +38,10 @@ action("generate_test_snapshot_bin") {
     rebase_path("$root_gen_dir"),
     "--script",
     rebase_path("test_snapshot.dart"),
+    "--vm_output_bin",
+    rebase_path(test_vm_isolate_snapshot, root_build_dir),
     "--output_bin",
-    rebase_path(output, root_build_dir),
+    rebase_path(test_isolate_snapshot, root_build_dir),
     "--target_os",
     os,
     "--url_mapping=dart:mojo.builtin,$builtin_path",
@@ -52,7 +56,8 @@ action("generate_test_snapshot_file") {
   inputs = [
     "//dart/runtime/tools/create_snapshot_file.py",
     "//mojo/dart/embedder/snapshot.cc.tmpl",
-    "$target_gen_dir/test_snapshot_gen.bin",
+    "$target_gen_dir/test_vm_isolate_snapshot.bin",
+    "$target_gen_dir/test_isolate_snapshot.bin",
   ]
   output = "$target_gen_dir/test_snapshot.cc"
   outputs = [
@@ -61,8 +66,10 @@ action("generate_test_snapshot_file") {
 
   script = "//dart/runtime/tools/create_snapshot_file.py"
   args = [
+    "--vm_input_bin",
+    rebase_path("$target_gen_dir/test_vm_isolate_snapshot.bin"),
     "--input_bin",
-    rebase_path("$target_gen_dir/test_snapshot_gen.bin"),
+    rebase_path("$target_gen_dir/test_isolate_snapshot.bin"),
     "--input_cc",
     rebase_path("//mojo/dart/embedder/snapshot.cc.tmpl"),
     "--output",