Handle escapes in string literals.

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 35 matching lines @@
   bool operator ==(other) {
     return other is SourceString && toString() == other.toString();
   }
 
   int get length() => end - begin;
 
   void printOn(StringBuffer sb) {
     sb.add(this);
   }
 
+  void printSubstringOn(StringBuffer sb, int from, int to) {
+    if (from >= to) return;
+    sb.add(internalString.substring(begin + from, begin + to));
+  }
+
+  int charCodeAt(int index) => internalString.charCodeAt(begin + index);
+
+  String operator[](int index) => internalString[begin + index];
+
   String toString() => internalString.substring(begin, end);
 
   String get stringValue() => toString();
 }