Don't run 'pub list-package-dirs' if there is no pubpsec.lock file.

pkg/analysis_server/test/source/caching_put_package_map_provider_test.dart

Index: pkg/analysis_server/test/source/caching_put_package_map_provider_test.dart
diff --git a/pkg/analysis_server/test/source/caching_put_package_map_provider_test.dart b/pkg/analysis_server/test/source/caching_put_package_map_provider_test.dart
index 2c1d2526be79979d22f1d1bc0335df02473550e5..9aa321b2a077287b2253917c0b34466efb4dc636 100644
--- a/pkg/analysis_server/test/source/caching_put_package_map_provider_test.dart
+++ b/pkg/analysis_server/test/source/caching_put_package_map_provider_test.dart
@@ -49,7 +49,9 @@ main() {
       for (String path in inputFiles) {
         resProvider.newFile(path, '');
       }
-      return resProvider.getResource(inputFiles[0]).parent;
+      Folder projectFolder = resProvider.getResource(inputFiles[0]).parent;
+      resProvider.newFile(projectFolder.path + '/pubspec.lock', '');
+      return projectFolder;
     }
 
     int mockWriteFile(File cacheFile, String content) {