anchor.diff:      @required bool isNonNullableByDefault,
     })  : _coreLibrary = coreLibrary,
           _asyncLibrary = asyncLibrary,
  -        _isNonNullableByDefault = isNonNullableByDefault;
  +        isNonNullableByDefault = isNonNullableByDefault;
   
     TypeProviderImpl get asLegacy {
  -    if (_isNonNullableByDefault) {
  +    if (isNonNullableByDefault) {
         return TypeProviderImpl(
           coreLibrary: _coreLibrary,
           asyncLibrary: _asyncLibrary,
  
positive.code:     return _nullElement ??= _getClassElement(_coreLibrary, 'Null');
    }
  
    InterfaceTypeImpl get nullStar {
      return _nullStar ??= nullElement.instantiate(
        typeArguments: const [],
  
negative.code:   VoidType get voidType => VoidTypeImpl.instance;
  
    NullabilitySuffix get _nullabilitySuffix {
      if (isNonNullableByDefault) {
        return NullabilitySuffix.none;
      } else {
        return NullabilitySuffix.star;
  