Fix ??= AST modification

tests/language/nullaware_opt_test.dart

 // Copyright (c) 2015, 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.
 //
 // VMOptions=--optimization_counter_threshold=5
 //
 // Basic null-aware operator test that invokes the optimizing compiler.
 
 import "package:expect/expect.dart";
 
@@ skipping 35 matching lines @@
   e?.f ??= 200;
   Expect.equals(200, e?.f);
 
   c?.f ??= 400;
   Expect.equals(null, c?.f);
   Expect.equals(null, c?.f++);
   e?.f++;
   Expect.equals(201, e.f);
 
   var x = 5 ?? bomb();
+  Expect.equals(5, x);
+
+  var y;
+  x = y ?? 1;
+  Expect.equals(1, x);
+  Expect.equals(null, y);
 }
 
 // Check that instructions without result do not crash.
 test2() {
   var c;
   c?.v;
   c?.m(bomb());
 }
 
 main() {
   for (int i = 0; i < 10; i++) {
     test();
     test2();
   }
 }