[CFE] Be explicit about invalidation strategy for flutter gallery leak tester

Before we added the --experiment flag when enabling because the
alternative invalidation strategy is not on by default.
This CL makes it always pass the --experiment flag (i.e. with the no-
version when we don't want it) making any default not matter.

Change-Id: Id62da7bcdee52f4556092a49e4939b9b66829d51
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/251265
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Jens Johansen <jensj@google.com>
diff --git a/pkg/front_end/test/flutter_gallery_leak_tester.dart b/pkg/front_end/test/flutter_gallery_leak_tester.dart
index 922bb31..68d066a 100644
--- a/pkg/front_end/test/flutter_gallery_leak_tester.dart
+++ b/pkg/front_end/test/flutter_gallery_leak_tester.dart
@@ -192,6 +192,8 @@
   ];
   if (alternativeInvalidation) {
     processArgs.add("--enable-experiment=alternative-invalidation-strategy");
+  } else {
+    processArgs.add("--enable-experiment=no-alternative-invalidation-strategy");
   }
 
   await heapHelper.start(processArgs,