[vm/dart2js] Add dart2js aot snapshot target to build scripts.

dart2js aot snapshot is used internally, so it makes sense to start building it externally to better understand its performance characteristics.
This CL doesn't add aot snapshot to dart sdk.

Change-Id: Id82a956138b118fb0a4bf0a16b1750e7bd4a801e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/330664
Reviewed-by: Siva Annamalai <[email protected]>
Commit-Queue: Alexander Aprelev <[email protected]>

Author: aam

utils/compiler/BUILD.gn

@@ -4,6 +4,7 @@
 
 import("../../sdk_args.gni")
 import("../../utils/compile_platform.gni")
+import("../aot_snapshot.gni")
 import("../application_snapshot.gni")
 import("../create_timestamp.gni")
 
@@ -77,6 +78,13 @@ application_snapshot("dart2js") {
   ]
 }
 
+aot_snapshot("dart2js_aot") {
+  deps = [ ":dart2js_create_snapshot_entry" ]
+
+  main_dart = "$target_gen_dir/dart2js.dart"
+  name = "dart2js_aot"
+}
+
 compile_platform("compile_dart2js_platform_unsound") {
   single_root_scheme = "org-dartlang-sdk"
   single_root_base = rebase_path("$sdk_root/")