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

dart/frog/leg/scanner/byte_strings.dart

Index: dart/frog/leg/scanner/byte_strings.dart
diff --git a/dart/frog/leg/scanner/byte_strings.dart b/dart/frog/leg/scanner/byte_strings.dart
index 17777fcafd5fa9e4e4cf79f0407b159d5f670fe0..e6daab14eda03bc13210f2d1de1598bbc50ac29c 100644
--- a/dart/frog/leg/scanner/byte_strings.dart
+++ b/dart/frog/leg/scanner/byte_strings.dart
@@ -15,9 +15,11 @@ class ByteString implements SourceString {
 
   abstract String get charset();
 
-  String toString() => new String.fromCharCodes(
+  String slowToString() => new String.fromCharCodes(
       new Utf8Decoder(bytes, offset, length).decodeRest());
 
+  String toString() => "ByteString(${slowToString()})";
+
   bool operator ==(other) {
     throw "should be overridden in subclass";
   }
@@ -41,7 +43,7 @@ class ByteString implements SourceString {
   }
 
   printOn(StringBuffer sb) {
-    sb.add(toString());
+    sb.add(slowToString());
   }
 
   bool isEmpty() => length == 0;