@J2ktIncompatible
  @GwtIncompatible // java serialization not supported in GWT.
  private static byte[] serializeWithBackReference(Object original, int handleOffset)
      throws IOException {
    ByteArrayOutputStream bos = new ByteArrayOutputStream();
    ObjectOutputStream out = new ObjectOutputStream(bos);

    out.writeObject(original);

    byte[] handle = toByteArray(baseWireHandle + handleOffset);

  @J2ktIncompatible
  @GwtIncompatible // java serialization not supported in GWT.
  private static byte[] serializeWithBackReference(Object original, int handleOffset)
      throws IOException {
    ByteArrayOutputStream bos = new ByteArrayOutputStream();
    ObjectOutputStream out = new ObjectOutputStream(bos);

    out.writeObject(original);

    byte[] handle = toByteArray(baseWireHandle + handleOffset);