void forEach(FontFaceSetForEachCallback callback(FontFace fontFace, FontFace fontFaceAgain, FontFaceSet set), [Object thisArg])

Source 

void forEach(FontFaceSetForEachCallback callback, [Object thisArg]) {
  if (thisArg != null) {
    _blink.BlinkFontFaceSet.instance.forEach_Callback_2_(this, callback, thisArg);
    return;
  }
  _blink.BlinkFontFaceSet.instance.forEach_Callback_1_(this, callback);
  return;
}