Windows/IFrames of different origin communicate with each other via the postMessage() mechanism. A script on a window can listen for incoming messages, but needs to know from where the message originated. This is possible by looking at [MessageEvent.source].

The following conceptual program works in dartium but not after compiling it with dart2js:

import 'dart:html';

main() {
  var iframe = new IFrameElement();
  iframe.src = "...";
  document.body.append(iframe);
  window.onMessage.listen((MessageEvent event) {
    if (event.source == iframe.contentWindow) {
      // This message is coming from [iframe].
      // => We get in here in dartium but not in dart2js.
    }
  });
}

The root cause is that the two _DOMWindowCrossFrame objects are not the same. There is a TODO in the following code to fix this:

dart/tools/dom/src/dart2js_DOMImplementation.dart:
 ...
 47 static WindowBase _createSafe(w) {
 48 if (identical(w, window)) {
 49 return w;
 50 } else {
 51 // TODO(vsm): Cache or implement equality.
 52 return new _DOMWindowCrossFrame(w);
 53 }
 54 }
  ...

@vsm/blois: Is there any chance someone could work on this? Would be highly appreciated.