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

sdk/lib/_internal/pub_generated/test/transformer/runs_a_third_party_transform_on_the_application_package_test.dart

+library pub_tests;
+import '../descriptor.dart' as d;
+import '../test_pub.dart';
+import '../serve/utils.dart';
+main() {
+  initConfig();
+  withBarbackVersions("any", () {
+    integration("runs a third-party transform on the application package", () {
+      d.dir(
+          "foo",
+          [
+              d.libPubspec("foo", '1.0.0'),
+              d.dir("lib", [d.file("foo.dart", REWRITE_TRANSFORMER)])]).create();
+      d.dir(appPath, [d.pubspec({
+          "name": "myapp",
+          "dependencies": {
+            "foo": {
+              "path": "../foo"
+            }
+          },
+          "transformers": ["foo"]
+        }), d.dir("web", [d.file("foo.txt", "foo")])]).create();
+      createLockFile('myapp', sandbox: ['foo'], pkg: ['barback']);
+      pubServe();
+      requestShouldSucceed("foo.out", "foo.out");
+      endPubServe();
+    });
+  });
+}