Fixing transferable_test

tests/html/transferables_test.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library TransferableTest;
 import '../../pkg/unittest/lib/unittest.dart';
 import '../../pkg/unittest/lib/html_config.dart';
 import 'dart:html';
 
 main() {
   useHtmlConfiguration();
 
   var isArrayBuffer =
       predicate((x) => x is ArrayBuffer, 'is an ArrayBuffer');
 
   test('TransferableTest', () {
-    window.onMessage.listen(expectAsync1((messageEvent) {
-      expect(messageEvent.data, isArrayBuffer);
-    }));
     final buffer = (new Float32Array(3)).buffer;
-    window.postMessage(buffer, '*', [buffer]);
+    window.postMessage({
+        'id': 'transferable data',
+        'buffer': buffer
+      }, '*', [buffer]);
+
+    return window.onMessage.firstMatching(
+      (e) {
+        return e.data is Map && e.data['id'] == 'transferable data';
+      }).then((messageEvent) {
+        expect(messageEvent.data['buffer'], isArrayBuffer);
+      });
   });
 }