code: function SHAWriteStream(manifest, filename, output) {
    this.output = output;
    this.manifest = manifest;
    this.filename = filename;
    this.sha = Crypto.createHash("sha1");
    output.on("close", this.emit.bind(this, "close"));
    output.on("error", this.emit.bind(this, "error"));
  }
docs: A write stream that calculates SHA from the output and updates the manifest accordingly.  manifest - Manifest object filename - Filename (manifest property to set) output   - Pipe to this output stream