Fix pub test that broke with changes in dart:convert

sdk/lib/_internal/pub/test/serve/404_page_test.dart

Index: sdk/lib/_internal/pub/test/serve/404_page_test.dart
diff --git a/sdk/lib/_internal/pub/test/serve/404_page_test.dart b/sdk/lib/_internal/pub/test/serve/404_page_test.dart
index d37c387caaab0e16efd434609c95f925fd37e014..7a77c408f959b4372bdba246eeab9451bf619487 100644
--- a/sdk/lib/_internal/pub/test/serve/404_page_test.dart
+++ b/sdk/lib/_internal/pub/test/serve/404_page_test.dart
@@ -39,7 +39,7 @@ main() {
       expect(response.statusCode, equals(404));
 
       // Should mention the asset that can't be found.
-      expect(response.body, contains('"/packages"'));
+      expect(response.body, contains('"/packages"'));
     });
 
     endPubServe();