MediaStream captureStream([num frameRate ])

Source 

MediaStream captureStream([num frameRate]) {
  if (frameRate != null) {
    return _blink.BlinkHTMLCanvasElement.instance
        .captureStream_Callback_1_(this, frameRate);
  }
  return _blink.BlinkHTMLCanvasElement.instance
      .captureStream_Callback_0_(this);
}