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

dart/frog/leg/warnings.dart

 // Copyright (c) 2011, 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 MessageKind {
   final String template;
   const MessageKind(this.template);
 
   static final GENERIC = const MessageKind('#{1}');
 
@@ skipping 124 matching lines @@
   final List arguments;
   String message;
 
   Message(this.kind, this.arguments);
 
   String toString() {
     if (message === null) {
       message = kind.template;
       int position = 1;
       for (var argument in arguments) {
-        message = message.replaceAll('#{${position++}}', argument.toString());
+        String string = slowToString(argument);
+        message = message.replaceAll('#{${position++}}', string);
       }
     }
     return message;
   }
 
   bool operator==(other) {
     if (other is !Message) return false;
     return (kind == other.kind) && (toString() == other.toString());
   }
+
+  String slowToString(object) {
+    if (object is SourceString) {
+      return object.slowToString();
+    } else {
+      return object.toString();
+    }
+  }
 }
 
 class TypeWarning {
   final Message message;
   TypeWarning.message(this.message);
   TypeWarning(MessageKind kind, List<Type> arguments)
     : message = new Message(kind, arguments);
   String toString() => message.toString();
 }
 
@@ skipping 13 matching lines @@
   String toString() => message.toString();
 }
 
 class CompileTimeConstantError {
   final Message message;
   CompileTimeConstantError.message(this.message);
   CompileTimeConstantError(MessageKind kind, List<Type> arguments)
     : message = new Message(kind, arguments);
   String toString() => message.toString();
 }