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

dart/frog/leg/universe.dart

Index: dart/frog/leg/universe.dart
diff --git a/dart/frog/leg/universe.dart b/dart/frog/leg/universe.dart
index 0353c744886e4709d72b491055bed13de8bb9399..6526aa0d615f9652e3f6f8fac05a9aca0bb677b5 100644
--- a/dart/frog/leg/universe.dart
+++ b/dart/frog/leg/universe.dart
@@ -142,7 +142,7 @@ class Invocation extends Selector {
 
     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;