void forEach(HeadersForEachCallback callback(String value, String key, Headers map), [Object thisArg])

Source 

void forEach(HeadersForEachCallback callback, [Object thisArg]) {
  if (thisArg != null) {
    _blink.BlinkHeaders.instance.forEach_Callback_2_(unwrap_jso(this), unwrap_jso((String value, String key, map) => callback(value, key, wrap_jso(map))), thisArg);
    return;
  }
  _blink.BlinkHeaders.instance.forEach_Callback_1_(unwrap_jso(this), unwrap_jso((String value, String key, map) => callback(value, key, wrap_jso(map))));
  return;
}