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

sdk/lib/_internal/pub_generated/test/build/outputs_error_to_json_test.dart

+library pub_tests;
+import 'package:scheduled_test/scheduled_test.dart';
+import '../../lib/src/exit_codes.dart' as exit_codes;
+import '../descriptor.dart' as d;
+import '../test_pub.dart';
+const TRANSFORMER = """
+import 'dart:async';
+
+import 'package:barback/barback.dart';
+
+class RewriteTransformer extends Transformer {
+  RewriteTransformer.asPlugin();
+
+  String get allowedExtensions => '.txt';
+
+  Future apply(Transform transform) => throw new Exception('oh no!');
+}
+""";
+main() {
+  initConfig();
+  withBarbackVersions("any", () {
+    integration("outputs error to JSON in a failed build", () {
+      currentSchedule.timeout *= 2;
+      d.dir(appPath, [d.pubspec({
+          "name": "myapp",
+          "transformers": ["myapp"]
+        }),
+            d.dir("lib", [d.file("transformer.dart", TRANSFORMER)]),
+            d.dir("web", [d.file("foo.txt", "foo")])]).create();
+      createLockFile('myapp', pkg: ['barback']);
+      schedulePub(args: ["build", "--format", "json"], outputJson: {
+        "buildResult": "failure",
+        "errors": [{
+            "error": startsWith(
+                "Transform Rewrite on myapp|web/foo.txt " "threw error: oh no!")
+          }],
+        "log": []
+      }, exitCode: exit_codes.DATA);
+    });
+  });
+}