Various scanner fixes:

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;
 
   StringScanner(String this.string) : super();
 
   int nextByte() => charAt(++byteOffset);
 
   int peek() => charAt(byteOffset + 1);
 
   int charAt(index)
       => (string.length > index) ? string.charCodeAt(index) : $EOF;
 
-  SourceString asciiString(int start) {
-    return new SubstringWrapper(string, start, byteOffset);
+  SourceString asciiString(int start, int offset) {
+    return new SubstringWrapper(string, start, byteOffset + offset);
   }
 
   SourceString utf8String(int start, int offset) {
     return new SubstringWrapper(string, start, byteOffset + offset + 1);
   }
 
   void appendByteStringToken(int kind, SourceString value) {
     // assert(kind != $a || keywords.get(value) == null);
     tail.next = new StringToken.fromSource(kind, value, tokenStart);
     tail = tail.next;
@@ skipping 15 matching lines @@
   }
 
   void printOn(StringBuffer sb) {
     sb.add(this);
   }
 
   String toString() => internalString.substring(begin, end);
 
   String get stringValue() => toString();
 }