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

dart/frog/leg/scanner/string_scanner.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.
 
 /**
  * Scanner that reads from a String and creates tokens that points to
  * substrings.
  */
 class StringScanner extends ArrayBasedScanner<SourceString> {
   final String string;
@@ skipping 23 matching lines @@
 }
 
 class SubstringWrapper implements SourceString {
   final String internalString;
   final int begin;
   final int end;
 
   const SubstringWrapper(String this.internalString,
                          int this.begin, int this.end);
 
-  int hashCode() => toString().hashCode();
+  int hashCode() => slowToString().hashCode();
 
   bool operator ==(other) {
-    return other is SourceString && toString() == other.toString();
+    return other is SourceString && slowToString() == other.slowToString();
   }
 
   void printOn(StringBuffer sb) {
-    sb.add(this);
+    sb.add(internalString.substring(begin, end));
   }
 
-  String toString() => internalString.substring(begin, end);
+  String slowToString() => internalString.substring(begin, end);
 
-  String get stringValue() => toString();
+  String toString() => "SubstringWrapper(slowToString())";
+
+  String get stringValue() => null;
 
   Iterator<int> iterator() =>
       new StringCodeIterator.substring(internalString, begin, end);
 
   SourceString copyWithoutQuotes(int initial, int terminal) {
     assert(0 <= initial);
     assert(0 <= terminal);
     assert(initial + terminal <= internalString.length);
     return new SubstringWrapper(internalString,
                                 begin + initial, end - terminal);
   }
 
   bool isEmpty() => begin == end;
 }