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

dart/frog/leg/warnings.dart

Index: dart/frog/leg/warnings.dart
diff --git a/dart/frog/leg/warnings.dart b/dart/frog/leg/warnings.dart
index 8d77a67d4bd862cd4ef88996ba559e705e3e09e0..4db77b47c7388615154e60d08f601e297b130017 100644
--- a/dart/frog/leg/warnings.dart
+++ b/dart/frog/leg/warnings.dart
@@ -142,7 +142,8 @@ class Message {
       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;
@@ -152,6 +153,14 @@ class Message {
     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 {