Fixing conversion of null event targets.

tools/dom/src/dart2js_Conversions.dart

Index: tools/dom/src/dart2js_Conversions.dart
diff --git a/tools/dom/src/dart2js_Conversions.dart b/tools/dom/src/dart2js_Conversions.dart
index 7e9ee41aaa11b51c96fcb5a961f021fb0fd2db69..8ced5efcaf5c6e5638eeeca0f9802db06cc12fe4 100644
--- a/tools/dom/src/dart2js_Conversions.dart
+++ b/tools/dom/src/dart2js_Conversions.dart
@@ -25,6 +25,9 @@ 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.