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

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

+library pub_tests;
+import '../descriptor.dart' as d;
+import '../test_pub.dart';
+import '../serve/utils.dart';
+main() {
+  initConfig();
+  withBarbackVersions("any", () {
+    integration("loads a transformer defined in an exported library", () {
+      d.dir(appPath, [d.pubspec({
+          "name": "myapp",
+          "transformers": ["myapp"]
+        }),
+            d.dir(
+                "lib",
+                [
+                    d.file("myapp.dart", "export 'src/transformer.dart';"),
+                    d.dir("src", [d.file("transformer.dart", REWRITE_TRANSFORMER)])]),
+            d.dir("web", [d.file("foo.txt", "foo")])]).create();
+      createLockFile('myapp', pkg: ['barback']);
+      pubServe();
+      requestShouldSucceed("foo.out", "foo.out");
+      endPubServe();
+    });
+  });
+}