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

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

+library pub_tests;
+import '../descriptor.dart' as d;
+import '../test_pub.dart';
+import '../serve/utils.dart';
+final transformer = """
+import 'dart:async';
+
+import 'package:barback/barback.dart';
+
+class RewriteTransformer extends Transformer {
+  RewriteTransformer.asPlugin();
+
+  String get allowedExtensions => '.txt';
+}
+""";
+main() {
+  initConfig();
+  withBarbackVersions("any", () {
+    integration("prints a transform interface error", () {
+      d.dir(appPath, [d.pubspec({
+          "name": "myapp",
+          "transformers": ["myapp/src/transformer"]
+        }),
+            d.dir("lib", [d.dir("src", [d.file("transformer.dart", transformer)])]),
+            d.dir("web", [d.file("foo.txt", "foo")])]).create();
+      createLockFile('myapp', pkg: ['barback']);
+      var server = pubServe();
+      server.stderr.expect(
+          emitsLines(
+              "Build error:\n" "Transform Rewrite on myapp|web/foo.txt threw error: Class "
+                  "'RewriteTransformer' has no instance method 'apply'."));
+      endPubServe();
+    });
+  });
+}