Close stdout and stderr before exiting Pub.

sdk/lib/_internal/pub/lib/src/command/list_package_dirs.dart

Index: sdk/lib/_internal/pub/lib/src/command/list_package_dirs.dart
diff --git a/sdk/lib/_internal/pub/lib/src/command/list_package_dirs.dart b/sdk/lib/_internal/pub/lib/src/command/list_package_dirs.dart
index f16ad65fd3be5877e4af37ee3ebed84db93e6006..5fbdcbfc8fe33f6bf082df2578f17a53bc62a345 100644
--- a/sdk/lib/_internal/pub/lib/src/command/list_package_dirs.dart
+++ b/sdk/lib/_internal/pub/lib/src/command/list_package_dirs.dart
@@ -12,6 +12,7 @@ import 'package:path/path.dart' as path;
 
 import '../command.dart';
 import '../exit_codes.dart' as exit_codes;
+import '../io.dart';
 import '../log.dart' as log;
 
 /// Handles the `list-package-dirs` pub command.
@@ -30,7 +31,7 @@ class ListPackageDirsCommand extends PubCommand {
     if (!entrypoint.lockFileExists) {
       log.error(JSON.encode(
           'Package "myapp" has no lockfile. Please run "pub install" first.'));
-      exit(exit_codes.NO_INPUT);
+      return flushThenExit(exit_codes.NO_INPUT);
     }
 
     var output = {};