dart2js should mangle/copy data when sending it through SendPorts. This is true, even when it sends to a SendPort that has its ReceivePort in the same isolate:

import 'dart:isolate';

main() {
  ReceivePort p = new ReceivePort();
  SendPort s = p.toSendPort();
  var t = [];
  t.add(t);
  p.receive((msg, _) {
    Expect.isFalse(identical(t, msg));
    p.close();
  });
  s.send(t);
}