gulp-retain

gRetain.js

function gRetain(options) {
  var stream = new Stream.Writable({objectMode:true})
    , fileMap = {}
    , timeout
    , size = 0
  ;

  options.limit = options.limit || 0;
  options.delay = options.delay || 300;

  stream._write = function(file, unused, cb) {
    size += file.contents.length
      - (fileMap[file.path] ? fileMap[file.path].contents.length : 0);
    if(options.limit && size > options.limit) {
      stream.emit('error', new Error('Maximum memory size reached!'));
      cb();
    }
    fileMap[file.path] = file;
    if(timeout) {
      clearTimeout(timeout);
    }
    timeout = setTimeout((function(files) {
      var newStream = new Stream.PassThrough({objectMode: true});
      function sendFile() {
        if(files.length) {
          newStream.write(files.shift.clone(), sendFile);
        } else {
          newStream.end();
        }
      }
      stream.emit('incoming', newStream);
      sendFile();
    })(Object.keys(fileMap).map(function(key) {
      return fileMap[key];
    })), options.delay);
    cb();
  }
  return stream;
}

// usage
gulp.src('coffee/**/*')
  .pipe(watch())
  .pipe(coffee())
  .pipe(gRetain())
  .on('incoming', function(stream) {
    stream.pipe(concat())
      .pipe(gulp.dest('build/'))
  })