Random lint cleanup (dart-lang/yaml_edit#112)

pkgs/yaml_edit/lib/src/errors.dart

diff --git a/pkgs/yaml_edit/lib/src/errors.dart b/pkgs/yaml_edit/lib/src/errors.dart
index 9b743a5..4a58440 100644
--- a/pkgs/yaml_edit/lib/src/errors.dart
+++ b/pkgs/yaml_edit/lib/src/errors.dart
@@ -85,7 +85,7 @@
   @override
   String toString() {
     if (message != null) {
-      return 'Assertion failed: ${message}';
+      return 'Assertion failed: $message';
     }
     return 'Assertion failed';
   }