Handle implicit closure of toString.

dart/frog/leg/lib/js_helper.dart

Index: dart/frog/leg/lib/js_helper.dart
diff --git a/dart/frog/leg/lib/js_helper.dart b/dart/frog/leg/lib/js_helper.dart
index ca78471a909b3163ed3598c2f1b89b0d453dd6d8..bc18132c541926c910afed5cc5b5cf1b23928c8e 100644
--- a/dart/frog/leg/lib/js_helper.dart
+++ b/dart/frog/leg/lib/js_helper.dart
@@ -1360,6 +1360,8 @@ interface Dynamic {
  */
 class Null {
   factory Null() {
-    throw UnsupportedOperationException('new Null()');
+    throw new UnsupportedOperationException('new Null()');
   }
 }
+
+builtin$get$toString(receiver) => () => builtin$toString$0(receiver);