Store the async-await compiled pub code directly in the repo.

sdk/lib/_internal/pub_generated/test/dart2js/converts_isolate_entrypoint_in_web_test.dart

+import 'package:scheduled_test/scheduled_test.dart';
+import '../descriptor.dart' as d;
+import '../test_pub.dart';
+import '../serve/utils.dart';
+main() {
+  initConfig();
+  integration("converts a Dart isolate entrypoint in web to JS", () {
+    currentSchedule.timeout *= 2;
+    d.dir(
+        appPath,
+        [
+            d.appPubspec(),
+            d.dir(
+                "web",
+                [
+                    d.file(
+                        "isolate.dart",
+                        "void main(List<String> args, SendPort "
+                            "sendPort) => print('hello');")])]).create();
+    pubServe();
+    requestShouldSucceed("isolate.dart.js", contains("hello"));
+    endPubServe();
+  });
+}