diff --git a/pkg/testing/lib/src/run.dart b/pkg/testing/lib/src/run.dart
index 49d0b1261ef..1930c457d49 100644
--- a/pkg/testing/lib/src/run.dart
+++ b/pkg/testing/lib/src/run.dart
@@ -196,6 +196,8 @@ class SuiteRunner {
     if (!hasRunnableTests) return null;
 
     return """
+// @dart=2.9
+
 library testing.generated;
 
 import 'dart:async' show Future;