function Chunk(root, next, taps) {
    this.root = root;
    this.next = next;
    this.data = []; //ie7 perf
    this.flushable = false;
    this.taps = taps;
  }
Pipes to a WritableStream. Note that backpressure isn't implemented,
so we just write as fast as we can.
@param stream {WritableStream}
@return self