Return null from stringValue and call slowToString() instead of toString().

dart/frog/leg/universe.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.
 
 class Universe {
   Map<Element, String> generatedCode;
   Map<Element, String> generatedBailoutCode;
   final Set<ClassElement> instantiatedClasses;
   final Set<FunctionElement> staticFunctionsNeedingGetter;
   final Map<SourceString, Set<Selector>> invokedNames;
@@ skipping 124 matching lines @@
         namedArguments = names,
         orderedNamedArguments = const <SourceString>[];
 
   List<SourceString> getOrderedNamedArguments() {
     if (namedArguments.isEmpty()) return namedArguments;
     // We use the empty const List as a sentinel.
     if (!orderedNamedArguments.isEmpty()) return orderedNamedArguments;
 
     List<SourceString> list = new List<SourceString>.from(namedArguments);
     list.sort((SourceString first, SourceString second) {
-      return first.stringValue.compareTo(second.stringValue);
+      return first.slowToString().compareTo(second.slowToString());
     });
     orderedNamedArguments = list;
     return orderedNamedArguments;
   }
 }