anchor.diff:  Object _computeNullable(type) {
     // *? normalizes to ?.
     if (_isLegacy(type)) {
  -    return nullable(JS<Object>('', '#.type', type));
  +    return nullable(JS<Object>('!', '#.type', type));
     }
     if (_isNullable(type) ||
         _isTop(type) ||
  
positive.code: 
  @notNull
  bool _isTop(type) {
    if (_isNullable(type)) return JS('!', '# === #', type.type, Object);
  
    return JS('!', '# === # || # === #', type, dynamic, type, void_);
  }
  
negative.code:   var result = isNormalized
        ? _Type(type)
        : (_isLegacy(type)
            ? wrapType(JS<Object>('!', '#.type', type))
            : _canonicalizeNormalizedTypeObject(type));
    JS('', '#[#] = #', type, _typeObject, result);
    return result;
  