VM: Fix bug with ??= expressions using await.

tests/language/await_null_aware_test.dart

Index: tests/language/await_null_aware_test.dart
diff --git a/tests/language/enum_index_test.dart b/tests/language/await_null_aware_test.dart
similarity index 51%
copy from tests/language/enum_index_test.dart
copy to tests/language/await_null_aware_test.dart
index 21a8e0e94442b2da9e6057547fb2b0be4d08b441..483d9dd396a9cf1e7a8f8e55fce969a605733c58 100644
--- a/tests/language/enum_index_test.dart
+++ b/tests/language/await_null_aware_test.dart
@@ -2,31 +2,22 @@
 // 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.
 
-// SharedOptions=--enable-enum
-
-// Test index access for enums.
-
-library enum_index_test;
+// Regression test for issue dartbug.com/24392
 
 import 'package:expect/expect.dart';
+import 'dart:async';
 
-enum Enum {
-  A,
-  B,
+Future<int> f() async {
+  // Unreachable.
+  Expect.isTrue(false);
 }
 
-class Class {
-  var index;
-}
+main() async {
+  int x = 1;
+  x ??= await f();
+  Expect.equals(1, x);
 
-main() {
-  test(null, new Class());
-  test(0, Enum.A);
-  test(1, Enum.B);
+  int y = 1;
+  y = y ?? await f();
+  Expect.equals(1, y);
 }
-
-test(expected, object) {
-  Expect.equals(expected, object.index);
-}
-
-