[testing] Opt out generated script in pkg/testing

Change-Id: I1ecb67ad5c7dbd712a2b36fbe8c5b8a1a6fd14f9
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/170340
Reviewed-by: Dmitry Stefantsov <dmitryas@google.com>
Commit-Queue: Johnni Winther <johnniwinther@google.com>

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;