anchor.diff:    RawReceivePort _packagesPort;
   
     void _requestPackagesMap([Uri packageConfig]) {
  -    assert(_packagesPort == null);
       assert(_rootScript != null);
  +    if (_packagesPort != null) {
  +      // Already scheduled.
  +      return;
  +    }
       // Create a port to receive the packages map on.
       _packagesPort = new RawReceivePort(_handlePackagesReply);
       var sp = _packagesPort.sendPort;
  
positive.code:     if (packagesConfigFlag != null) {
        _setPackagesConfig(packagesConfigFlag);
      }
      _fileRequestQueue = new List<FileRequest>();
    }
  
    void cleanup() {
      if (_packagesPort != null) {
        _packagesPort.close();
        _packagesPort = null;
  
negative.code:   }
  
    void _handlePackagesReply(msg) {
      if (_packagesPort == null) {
        return;
      }
      // Make sure to close the _packagePort before any other action.
      _packagesPort.close();
      _packagesPort = null;
  