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

dart/frog/leg/scanner/scanner.dart

 // Copyright (c) 2012, 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.
 
 interface Scanner {
   Token tokenize();
 }
 
 /**
  * Common base class for a Dart scanner.
@@ skipping 571 matching lines @@
           appendByteStringToken(IDENTIFIER_INFO, asciiString(start, 0));
         } else {
           appendByteStringToken(IDENTIFIER_INFO, utf8String(start, -1));
         }
         return next;
       } else {
         int nonAsciiStart = byteOffset;
         do {
           next = nextByte();
         } while (next > 127);
-        String string = utf8String(nonAsciiStart, -1).toString();
+        String string = utf8String(nonAsciiStart, -1).slowToString();
         isAscii = false;
         int byteLength = nonAsciiStart - byteOffset;
         addToCharOffset(string.length - byteLength);
       }
     }
   }
 
   int tokenizeRawString(int next) {
     int start = byteOffset;
     next = advance();
@@ skipping 145 matching lines @@
                                       charOffset);
   }
 }
 
 class MalformedInputException {
   final String message;
   final position;
   MalformedInputException(this.message, this.position);
   toString() => message;
 }