pkg/front_end/testcases/runtime_checks_new/contravariant_combiner.dart:22:44: Context: Write to b@535
  b /*@checkReturn=B<num, (num) -> void>*/ += 1;
                                           ^^
pkg/front_end/testcases/runtime_checks_new/contravariant_combiner.dart:23:52: Context: Write to b@535
  var x = b /*@checkReturn=B<num, (num) -> void>*/ += 2;
                                                   ^^