Fixing conversion of null event targets.

sdk/lib/html/dart2js/html_dart2js.dart

Index: sdk/lib/html/dart2js/html_dart2js.dart
diff --git a/sdk/lib/html/dart2js/html_dart2js.dart b/sdk/lib/html/dart2js/html_dart2js.dart
index 018b8aa93a10c2ead94f064616a433dc50934513..d049073721d8d9c10f3563a677846d6935f8311d 100644
--- a/sdk/lib/html/dart2js/html_dart2js.dart
+++ b/sdk/lib/html/dart2js/html_dart2js.dart
@@ -32789,7 +32789,6 @@ class _PureIsolateTimer implements Timer {
     _sendPort = _port.toSendPort();
     _port.receive((msg, replyTo) {
       assert(msg == _TIMER_PING);
-      assert(replyTo == _HELPER_ISOLATE_PORT);
       callback(this);
       if (!repeating) _cancel();
     });
@@ -33619,6 +33618,9 @@ WindowBase _convertNativeToDart_Window(win) {
 }
 
 EventTarget _convertNativeToDart_EventTarget(e) {
+  if (e == null) {
+    return null;
+  }
   // Assume it's a Window if it contains the setInterval property.  It may be
   // from a different frame - without a patched prototype - so we cannot
   // rely on Dart type checking.