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

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

+library pub_tests;
+import 'package:scheduled_test/scheduled_test.dart';
+import '../descriptor.dart' as d;
+import '../test_pub.dart';
+import '../serve/utils.dart';
+const REJECT_CONFIG_TRANSFORMER = """
+import 'dart:async';
+
+import 'package:barback/barback.dart';
+
+class RejectConfigTransformer extends Transformer {
+  RejectConfigTransformer.asPlugin(BarbackSettings settings) {
+    throw "I hate these settings!";
+  }
+
+  Future<bool> isPrimary(_) => new Future.value(true);
+  Future apply(Transform transform) {}
+}
+""";
+main() {
+  initConfig();
+  withBarbackVersions("any", () {
+    integration("a transformer can reject is configuration", () {
+      d.dir(appPath, [d.pubspec({
+          "name": "myapp",
+          "transformers": [{
+              "myapp/src/transformer": {
+                'foo': 'bar'
+              }
+            }]
+        }),
+            d.dir(
+                "lib",
+                [
+                    d.dir(
+                        "src",
+                        [d.file("transformer.dart", REJECT_CONFIG_TRANSFORMER)])])]).create();
+      createLockFile('myapp', pkg: ['barback']);
+      var pub = startPubServe();
+      pub.stderr.expect(
+          endsWith('Error loading transformer: I hate these ' 'settings!'));
+      pub.shouldExit(1);
+    });
+  });
+}