async report(_folder) {
    const folder = _folder || this.istanbulFolder;

    const collector = new istanbul.Collector();
    const reporter = new istanbul.Reporter(false, folder);

    return new Promise((resolve, reject) => {
      try {
        this.coverage.generate(this.instrumenter.instrumentationData);

        const mapping = this.makeKeysRelative(this.coverage.data, this.cwd);
        this.saveCoverage(mapping);

        collector.add(mapping);

        this.istanbulReporter.forEach(report => reporter.add(report));

        // Pify doesn't like this one...
        reporter.write(collector, true, (err) => {
          if (err) return reject(err);

          this.ui.report('istanbul');
          resolve();
        });

      } catch (error) {
        error.message = this.ui.generate('istanbul-fail') + error.message;
        throw error;
      }
    })
  }
Generate coverage / write coverage report / run istanbul