Fix serialization of IndexSetIfNullStructure.

tests/compiler/dart2js/serialization/test_data.dart

Index: tests/compiler/dart2js/serialization/test_data.dart
diff --git a/tests/compiler/dart2js/serialization/test_data.dart b/tests/compiler/dart2js/serialization/test_data.dart
index 6dfa0955e80f85ede5e2c807c8acd326a48cc1f9..d27ec305fd72e4bae5aea5c7d3cbd4fc2b4b0031 100644
--- a/tests/compiler/dart2js/serialization/test_data.dart
+++ b/tests/compiler/dart2js/serialization/test_data.dart
@@ -454,6 +454,18 @@ class A {
 }
 ''',
   }),
+
+  const Test('Index set if null', const {
+    'main.dart': '''
+import 'a.dart';
+
+main() => m(null, null, null);
+''',
+  }, preserializedSourceFiles: const {
+    'a.dart': '''
+m(a, b, c) => a[b] ??= c;
+''',
+  }),
 ];
 
 class Test {