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

dart/frog/leg/scanner/byte_strings.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.
 
 /**
  * An abstract string representation.
  */
 class ByteString implements SourceString {
   final List<int> bytes;
   final int offset;
   final int length;
   int _hashCode;
 
   ByteString(List<int> this.bytes, int this.offset, int this.length);
 
   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";
   }
 
   Iterator<int> iterator() => new Utf8Decoder(bytes, offset, length);
 
   int hashCode() {
     if (_hashCode === null) {
       _hashCode = computeHashCode();
     }
     return _hashCode;
   }
 
   int computeHashCode() {
     int code = 1;
     int end = offset + length;
     for (int i = offset; i < end; i++) {
       code += 19 * code + bytes[i];
     }
     return code;
   }
 
   printOn(StringBuffer sb) {
-    sb.add(toString());
+    sb.add(slowToString());
   }
 
   bool isEmpty() => length == 0;
 }
 
 /**
  * A string that consists purely of 7bit ASCII characters.
  */
 class AsciiString extends ByteString {
   final String charset = "ASCII";
@@ skipping 75 matching lines @@
  * A ByteString-valued token.
  */
 class ByteStringToken extends Token {
   final ByteString value;
 
   ByteStringToken(PrecedenceInfo info, ByteString this.value, int charOffset)
     : super(info, charOffset);
 
   String toString() => value.toString();
 }