Include file path in JSON error when possible.

sdk/lib/_internal/pub/test/list_package_dirs/lockfile_error_test.dart

Index: sdk/lib/_internal/pub/test/list_package_dirs/lockfile_error_test.dart
diff --git a/sdk/lib/_internal/pub/test/list_package_dirs/missing_pubspec_test.dart b/sdk/lib/_internal/pub/test/list_package_dirs/lockfile_error_test.dart
similarity index 56%
copy from sdk/lib/_internal/pub/test/list_package_dirs/missing_pubspec_test.dart
copy to sdk/lib/_internal/pub/test/list_package_dirs/lockfile_error_test.dart
index da2feb35130202af39eb43d6f128f50b896ee4c0..893e20679480d004117789c1c26b7635d2844754 100644
--- a/sdk/lib/_internal/pub/test/list_package_dirs/missing_pubspec_test.dart
+++ b/sdk/lib/_internal/pub/test/list_package_dirs/lockfile_error_test.dart
@@ -3,6 +3,7 @@
 // BSD-style license that can be found in the LICENSE d.file.
 
 import 'package:path/path.dart' as path;
+import 'package:scheduled_test/scheduled_test.dart';
 
 import '../../lib/src/exit_codes.dart' as exit_codes;
 import '../../lib/src/io.dart';
@@ -11,16 +12,17 @@ import '../test_pub.dart';
 
 main() {
   initConfig();
-  // This is a regression test for #20065.
-  integration("reports a missing pubspec error using JSON", () {
-    d.dir(appPath).create();
+  integration("reports the lockfile path when there is an error in it", () {
+    d.dir(appPath, [
+      d.appPubspec(),
+      d.file("pubspec.lock", "some bad yaml")
+    ]).create();
 
     schedulePub(args: ["list-package-dirs", "--format=json"],
         outputJson: {
-          "error":
-            'Could not find a file named "pubspec.yaml" in "'
-                '${canonicalize(path.join(sandboxDir, appPath))}".'
+          "error": contains('The lockfile must be a YAML mapping.'),
+          "path": canonicalize(path.join(sandboxDir, appPath, "pubspec.lock"))
         },
-        exitCode: 1);
+        exitCode: exit_codes.DATA);
   });
 }