Fix incorrect nullability inference for ??

tests/language_strong/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 61 matching lines @@
   String s;
 }
 
 class Foo {
   Bar _bar;
   String str;
 
   Foo(this._bar) : str = _bar?.s;
 }
 
+// Check that ?? isn't incorrectly optimized as non-nullable
+// (DDC regression test)
+test3() {
+  List n = null;
+  var func = n?.add;
+  var result = func ?? 1;
+  Expect.equals(result, 1);
+}
+
 main() {
   for (int i = 0; i < 10; i++) {
     test();
     test2();
+    test3();
   }
 
   Expect.equals(null, new Foo(new Bar()).str);
 }