Fix serialization of IndexSetIfNullStructure.

tests/compiler/dart2js/serialization/test_data.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library dart2js.serialization_test_data;
 
 const List<Test> TESTS = const <Test>[
   const Test('Empty program', const {
     'main.dart': 'main() {}'
   }),
@@ skipping 436 matching lines @@
 }
 ''',
   }, preserializedSourceFiles: const {
     'a.dart': '''
 class A {
   final field1;
   const A(a) : this.field1 = a + 1.0;
 }
 ''',
   }),
+
+  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 {
   final String name;
   final Map sourceFiles;
   final Map preserializedSourceFiles;
   final Map unserializedSourceFiles;
   final int expectedErrorCount;
   final int expectedWarningCount;
   final int expectedHintCount;
   final int expectedInfoCount;
 
   const Test(
       this.name,
       this.sourceFiles,
       {this.preserializedSourceFiles,
       this.unserializedSourceFiles,
       this.expectedErrorCount: 0,
       this.expectedWarningCount: 0,
       this.expectedHintCount: 0,
       this.expectedInfoCount: 0});
 }