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

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

+import 'package:scheduled_test/scheduled_test.dart';
+import '../descriptor.dart' as d;
+import '../test_pub.dart';
+import '../serve/utils.dart';
+main() {
+  initConfig();
+  withBarbackVersions("any", () {
+    integration(
+        "compiles a Dart file that imports a generated file in another "
+            "package to JS",
+        () {
+      d.dir("foo", [d.pubspec({
+          "name": "foo",
+          "version": "0.0.1",
+          "transformers": ["foo/transformer"]
+        }), d.dir("lib", [d.file("foo.dart", """
+library foo;
+const TOKEN = "before";
+foo() => TOKEN;
+"""), d.file("transformer.dart", dartTransformer("munge"))])]).create();
+      d.dir(appPath, [d.appPubspec({
+          "foo": {
+            "path": "../foo"
+          }
+        }), d.dir("web", [d.file("main.dart", """
+import "package:foo/foo.dart";
+main() => print(foo());
+""")])]).create();
+      createLockFile("myapp", sandbox: ["foo"], pkg: ["barback"]);
+      pubServe();
+      requestShouldSucceed("main.dart.js", contains("(before, munge)"));
+      endPubServe();
+    });
+  });
+}