Attempt to solve the flakiness of the test.

tests/html/native_gc_test.dart

 library NativeGCTest;
 import '../../pkg/unittest/lib/unittest.dart';
 import '../../pkg/unittest/lib/html_config.dart';
 import 'dart:html';
 
 main() {
   useHtmlConfiguration();
 
   test('EventListener', () {
       final int N = 1000000;
@@ skipping 14 matching lines @@
             // Note: the reference to l keeps the entire list alive.
             expect(l[N - 1], M - 1);
           }, false);
       }
 
       final event = new Event('test');
       div.on['test'].dispatch(event);
   });
 
   test('WindowEventListener', () {
+    String message = 'WindowEventListenerTestPingMessage';
+
     Element testDiv = new DivElement();
     testDiv.id = '#TestDiv';
     document.body.nodes.add(testDiv);
-    window.on.message.add((e) => testDiv.click());
+    window.on.message.add((e) {
+      if (e.data == message) testDiv.click();
+    });
 
     for (int i = 0; i < 100; ++i) {
       triggerMajorGC();
     }
 
     testDiv.on.click.add(expectAsync1((e) {}));
-    window.postMessage('test', '*');
+    window.postMessage(message, '*');
   });
 }
 
 void triggerMajorGC() {
   List list = new List.fixedLength(1000000);
   Element div = new DivElement();
   div.on.click.add((e) => print(list[0]));
 }