How to use the @stryker-mutator/util.fsAsPromised.readFile function in @stryker-mutator/util

private readFile(fileName: string): Promise {
    return fsAsPromised
      .readFile(fileName)
      .then((content: Buffer) => new File(fileName, content))
      .then((file: File) => {
        this.reportSourceFilesRead(file);
        return file;
      })
      .catch(error => {
        if (
          (isErrnoException(error) && error.code === 'ENOENT') ||
          error.code === 'EISDIR'
        ) {
          return null; // file is deleted or a directory. This can be a valid result of the git command
        } else {
          // Rethrow
          throw error;
        }

private readFile(fileName: string): Promise {
    return fsAsPromised
      .readFile(fileName)
      .then((content: Buffer) => new File(fileName, content))
      .then((file: File) => {
        this.reportSourceFilesRead(file);
        return file;
      })
      .catch(error => {
        if ((isErrnoException(error) && error.code === 'ENOENT') || error.code === 'EISDIR') {
          return null; // file is deleted or a directory. This can be a valid result of the git command
        } else {
          // Rethrow
          throw error;
        }
      });
  }
}

export async function readMutationTestResult(eventResultDirectory = path.resolve('reports', 'mutation', 'events')) {
  const allReportFiles = await fsAsPromised.readdir(eventResultDirectory);
  const mutationTestReportFile = allReportFiles.find(file => !!file.match(/.*onMutationTestReportReady.*/));
  expect(mutationTestReportFile).ok;
  const mutationTestReportContent = await fsAsPromised.readFile(path.resolve(eventResultDirectory, mutationTestReportFile || ''), 'utf8');
  const report = JSON.parse(mutationTestReportContent) as mutationTestReportSchema.MutationTestResult;
  const metricsResult = calculateMetrics(report.files);
  return metricsResult;
}