Use write request?

[stevenvachon]

As per mochajs/mocha#333 (comment):

runner = mocha.run(!program.exit ? exitLater : function(code){
  var draining = 0;
  var exit = function(){ if (!(draining--)) process.exit(code); };
  var streams = [process.stdout, process.stderr];
  streams.forEach(function(stream){
    // submit empty write request and wait for completion
    draining += 1;
    stream.write('', exit);
  });
  exit();
});