Bugfix in html_dart2js: Detect window objects in a way compatible with cross domain iframes

dart/sdk/lib/html/dart2js/html_dart2js.dart

Index: dart/sdk/lib/html/dart2js/html_dart2js.dart
diff --git a/dart/sdk/lib/html/dart2js/html_dart2js.dart b/dart/sdk/lib/html/dart2js/html_dart2js.dart
index 7a2e9050709f9a44b31f75fa7ede832280a597d0..d65d60e8904eca3ac442c4777f3289ba812a5011 100644
--- a/dart/sdk/lib/html/dart2js/html_dart2js.dart
+++ b/dart/sdk/lib/html/dart2js/html_dart2js.dart
@@ -35436,10 +35436,10 @@ EventTarget _convertNativeToDart_EventTarget(e) {
   if (e == null) {
     return null;
   }
-  // Assume it's a Window if it contains the setInterval property.  It may be
+  // Assume it's a Window if it contains the postMessage property.  It may be
   // from a different frame - without a patched prototype - so we cannot
   // rely on Dart type checking.
-  if (JS('bool', r'"setInterval" in #', e)) {
+  if (JS('bool', r'"postMessage" in #', e)) {
     var window = _DOMWindowCrossFrame._createSafe(e);
     // If it's a native window.
     if (window is EventTarget) {