toStringAsExponential allows the fractionDigits argument to be null.

frog/leg/lib/js_helper.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #library('js_helper');
 
 #import('coreimpl.dart');
 
 #source('constant_map.dart');
 #source('date_helper.dart');
@@ skipping 950 matching lines @@
   String result = JS('String', @'$0.toFixed($1)', receiver, fractionDigits);
   if (receiver == 0 && receiver.isNegative()) return "-$result";
   return result;
 }
 
 builtin$toStringAsExponential$1(receiver, fractionDigits) {
   checkNull(receiver);
   if (receiver is !num) {
     return UNINTERCEPTED(receiver.toStringAsExponential(fractionDigits));
   }
-  checkNum(fractionDigits);
+  if (fractionDigits !== null) checkNum(fractionDigits);
 
   String result = JS('String', @'$0.toExponential($1)',
                      receiver, fractionDigits);
   if (receiver == 0 && receiver.isNegative()) return "-$result";
   return result;
 }
 
 builtin$toStringAsPrecision$1(receiver, fractionDigits) {
   checkNull(receiver);
   if (receiver is !num) {
@@ skipping 416 matching lines @@
 // Called at the end of unaborted switch cases to get the (unique)
 // FallThroughError exception that will be thrown.
 FallThroughError fallThroughErrorInstance = null;
 getFallThroughError() {
   // TODO(lrn): Redo when we support "const FallThroughError()".
   if (fallThroughErrorInstance === null) {
     fallThroughErrorInstance = new FallThroughError();
   }
   return fallThroughErrorInstance;
 }