pkg/front_end/testcases/unsound_promotion.dart:20:9: Context: Possible promotion of s@397
  if (s is A) {
        ^^
pkg/front_end/testcases/unsound_promotion.dart:31:9: Context: Write to aList@541
  aList = list;
        ^
pkg/front_end/testcases/unsound_promotion.dart:33:9: Context: Write to aList@541
  aList = o;
        ^