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

sdk/lib/_internal/pub_generated/test/serve/supports_user_defined_lazy_transformers_test.dart

+library pub_tests;
+import '../descriptor.dart' as d;
+import '../test_pub.dart';
+import 'utils.dart';
+main() {
+  initConfig();
+  withBarbackVersions("any", () {
+    integration("supports a user-defined lazy transformer", () {
+      d.dir(appPath, [d.pubspec({
+          "name": "myapp",
+          "transformers": ["myapp/src/transformer"]
+        }),
+            d.dir("lib", [d.dir("src", [d.file("transformer.dart", LAZY_TRANSFORMER)])]),
+            d.dir("web", [d.file("foo.txt", "foo")])]).create();
+      createLockFile('myapp', pkg: ['barback']);
+      var server = pubServe();
+      server.stdout.expect('Build completed successfully');
+      requestShouldSucceed("foo.out", "foo.out");
+      server.stdout.expect(
+          emitsLines('[Info from LazyRewrite]:\n' 'Rewriting myapp|web/foo.txt.'));
+      endPubServe();
+    });
+  });
+}