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

sdk/lib/_internal/pub_generated/test/list_package_dirs/missing_pubspec_test.dart

+import 'package:path/path.dart' as path;
+import '../../lib/src/io.dart';
+import '../descriptor.dart' as d;
+import '../test_pub.dart';
+main() {
+  initConfig();
+  integration("reports a missing pubspec error using JSON", () {
+    d.dir(appPath).create();
+    schedulePub(args: ["list-package-dirs", "--format=json"], outputJson: {
+      "error": 'Could not find a file named "pubspec.yaml" in "'
+          '${canonicalize(path.join(sandboxDir, appPath))}".',
+      "path": canonicalize(path.join(sandboxDir, appPath, "pubspec.yaml"))
+    }, exitCode: 1);
+  });
+}