void toBlob(BlobCallback callback, String type, [ Object arguments ])

Source 

void toBlob(BlobCallback callback, String type, [Object arguments]) {
  if (arguments != null) {
    _blink.BlinkHTMLCanvasElement.instance
        .toBlob_Callback_3_(this, callback, type, arguments);
    return;
  }
  _blink.BlinkHTMLCanvasElement.instance
      .toBlob_Callback_2_(this, callback, type);
  return;
}