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