cleanup to Value - fix for StringEscapesTest

frog/token.dart

Index: frog/token.dart
diff --git a/frog/token.dart b/frog/token.dart
index 4d46117e33d73bede841b4b5b8d0fe04f0b7df22..8e5b98c7044088b35ca7ddf5fc60e53b7873d2fb 100644
--- a/frog/token.dart
+++ b/frog/token.dart
@@ -45,6 +45,13 @@ class Token {
   }
 }
 
+/** A token containing a parsed literal value. */
+class LiteralToken extends Token {
+  var value;
+  LiteralToken(kind, source, start, end, this.value)
+      : super(kind, source, start, end);
+}
+
 /** A token containing error information. */
 class ErrorToken extends Token {
   String message;