docs: function checkEnvironment(expected) {
    exec('yarn --version', function(yarnErr, yarnStdout) {
      var actualNodeVersion = process.version;
      var actualYarnVersion = !yarnErr && semver.clean(yarnStdout);
      var issues = [];
  
      // Check Node version.
      if (!semver.satisfies(actualNodeVersion, expected.nodeVersion)) {
        issues.push(
            'You are running an unsupported Node version. Expected: ' + expected.nodeVersion +
            ' Found: ' + actualNodeVersion + '. Use nvm to update your Node version.');
      }
  
      // Check yarn version.
      if (yarnErr) {
        issues.push(
            'You don\'t have yarn globally installed. This is required if you want to work on this ' +
            'project. Installation instructions: https://yarnpkg.com/lang/en/docs/install/');
      } else if (!semver.satisfies(actualYarnVersion, expected.yarnVersion)) {
        issues.push(
            'You are running an unsupported yarn version. Expected: ' + expected.yarnVersion +
            ' Found: ' + actualYarnVersion + '. For instructions see:' +
            ' https://yarnpkg.com/lang/en/docs/install/');
      }
  
      reportIssues(issues);
    });
  }