Fix incorrect nullability inference for ??

tests/language_strong/nullaware_opt_test.dart

Index: tests/language_strong/nullaware_opt_test.dart
diff --git a/tests/language_strong/nullaware_opt_test.dart b/tests/language_strong/nullaware_opt_test.dart
index 0436e8b2aac64106d1412e233176264a36566ad2..017ae4c433cef6885a5e7101a427f24e81779e98 100644
--- a/tests/language_strong/nullaware_opt_test.dart
+++ b/tests/language_strong/nullaware_opt_test.dart
@@ -79,10 +79,20 @@ class Foo {
   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);