Fixing transferable_test

tests/html/transferables_test.dart

Index: tests/html/transferables_test.dart
diff --git a/tests/html/transferables_test.dart b/tests/html/transferables_test.dart
index 3db9c42641cc8113e9ea13ebc5510c9bb366a282..f7f4f475fcdd1921d3e07d630bbe1eae975c9f7b 100644
--- a/tests/html/transferables_test.dart
+++ b/tests/html/transferables_test.dart
@@ -14,10 +14,17 @@ main() {
       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);
+      });
   });
 }