anchor.diff:    const factory TypeMask.empty() = FlatTypeMask.empty;
   
     factory TypeMask.exact(ClassEntity base, ClosedWorld closedWorld) {
  -    assert(invariant(base, closedWorld.isInstantiated(base),
  -        message: () => "Cannot create exact type mask for uninstantiated "
  +    assert(
  +        closedWorld.isInstantiated(base),
  +        failedAt(
  +            base,
  +            "Cannot create exact type mask for uninstantiated "
               "class $base.\n${closedWorld.dump(base)}"));
       return new FlatTypeMask.exact(base);
     }
  
positive.code:   const factory TypeMask.nonNullEmpty() = FlatTypeMask.nonNullEmpty;
  
    factory TypeMask.nonNullExact(ClassEntity base, ClosedWorld closedWorld) {
      assert(invariant(base, closedWorld.isInstantiated(base),
          message: () => "Cannot create exact type mask for uninstantiated "
              "class $base.\n${closedWorld.dump(base)}"));
      return new FlatTypeMask.nonNullExact(base);
    }
  
negative.code:    */
    static bool assertIsNormalized(TypeMask mask, ClosedWorld closedWorld) {
      String reason = getNotNormalizedReason(mask, closedWorld);
      assert(reason == null,
          failedAt(NO_LOCATION_SPANNABLE, '$mask is not normalized: $reason'));
      return true;
    }
  
    static String getNotNormalizedReason(TypeMask mask, ClosedWorld closedWorld) {
  