void setSelectionRange(int start, int end, [ String direction ])

Source 

void setSelectionRange(int start, int end, [String direction]) {
  if (direction != null) {
    _blink.BlinkHTMLTextAreaElement.instance
        .setSelectionRange_Callback_3_(this, start, end, direction);
    return;
  }
  _blink.BlinkHTMLTextAreaElement.instance
      .setSelectionRange_Callback_2_(this, start, end);
  return;
}