Fix ??= AST modification

tests/language/nullaware_opt_test.dart

Index: tests/language/nullaware_opt_test.dart
diff --git a/tests/language/nullaware_opt_test.dart b/tests/language/nullaware_opt_test.dart
index 5446171d775930479cf74f11f49eaf5ff4a7cb35..e83757a9190cd22707809fa44eef54c36411f25d 100644
--- a/tests/language/nullaware_opt_test.dart
+++ b/tests/language/nullaware_opt_test.dart
@@ -53,6 +53,12 @@ test() {
   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.