DataWriter = function(opt_size) {
  var loc = 0;
  var view = new Uint8Array(new ArrayBuffer(opt_size || 512));

  this.byte_ = function(v) {
    view[loc] = v;
    ++loc;
    this.buffer = view.buffer.slice(0, loc);
  }.bind(this);
}
DataWriter writes data to an ArrayBuffer, presenting it as the instance
variable 'buffer'.

@constructor