anchor.diff:    // The map describing how certain package names are mapped to Uris.
     Uri _packageConfig = null;
     Map<String, Uri> _packageMap = null;
  +
  +  // We issue only 16 concurrent calls to File.readAsBytes() to stay within
  +  // platform-specific resource limits (e.g. max open files). The rest go on
  +  // _fileRequestQueue and are processed when we can safely issue them.
  +  static const int _maxFileRequests = 16;
  +  int currentFileRequests = 0;
  +  List<FileRequest> _fileRequestQueue;
  +
  +  bool get shouldIssueFileRequest => currentFileRequests < _maxFileRequests;
  +  void enqueueFileRequest(FileRequest fr) {
  +    _fileRequestQueue.add(fr);
  +  }
  +  FileRequest dequeueFileRequest() {
  +    if (_fileRequestQueue.length == 0) {
  +      return null;
  +    }
  +    return _fileRequestQueue.removeAt(0);
  +  }
   
     _setPackageRoot(String packageRoot) {
       packageRoot = _sanitizeWindowsPath(packageRoot);
  
positive.code:   return uri;
  }
  
  // State associated with the isolate that is used for loading.
  class IsolateLoaderState extends IsolateEmbedderData {
    IsolateLoaderState(this.isolateId);
  
negative.code:                        Uri resolvedUri,
                         String libraryUrl) {
    if (resolvedUri.scheme == '' || resolvedUri.scheme == 'file') {
      if (loaderState.shouldIssueFileRequest) {
        _loadFile(loaderState, sp, tag, uri, resolvedUri, libraryUrl);
        loaderState.currentFileRequests++;
      } else {
        FileRequest fr = new FileRequest(sp, tag, uri, resolvedUri, libraryUrl);
        loaderState.enqueueFileRequest(fr);
      }
    } else if ((resolvedUri.scheme == 'http') ||
               (resolvedUri.scheme == 'https')) {
      _loadHttp(sp, tag, uri, resolvedUri, libraryUrl);
  