toStringAsExponential allows the fractionDigits argument to be null.

frog/leg/lib/js_helper.dart

Index: frog/leg/lib/js_helper.dart
diff --git a/frog/leg/lib/js_helper.dart b/frog/leg/lib/js_helper.dart
index 85321333b5f72d16402818e4808a311383e9509e..9cc4636b8d71a47d8a50f956c93fba768557af76 100644
--- a/frog/leg/lib/js_helper.dart
+++ b/frog/leg/lib/js_helper.dart
@@ -968,7 +968,7 @@ builtin$toStringAsExponential$1(receiver, fractionDigits) {
   if (receiver is !num) {
     return UNINTERCEPTED(receiver.toStringAsExponential(fractionDigits));
   }
-  checkNum(fractionDigits);
+  if (fractionDigits !== null) checkNum(fractionDigits);
 
   String result = JS('String', @'$0.toExponential($1)',
                      receiver, fractionDigits);