Various scanner fixes:

dart/frog/leg/scanner/byte_array_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 byte array and creates tokens that points
  * to the same array.
  */
 class ByteArrayScanner extends ArrayBasedScanner<ByteString> {
   final List<int> bytes;
 
   ByteArrayScanner(List<int> this.bytes) : super();
 
   int nextByte() => byteAt(++byteOffset);
 
   int peek() => byteAt(byteOffset + 1);
 
   int byteAt(int index) => bytes[index];
 
-  AsciiString asciiString(int start) {
-    return AsciiString.of(bytes, start, byteOffset - start);
+  AsciiString asciiString(int start, int offset) {
+    return AsciiString.of(bytes, start, byteOffset - start + offset);
   }
 
   Utf8String utf8String(int start, int offset) {
     return Utf8String.of(bytes, start, byteOffset - start + offset + 1);
   }
 
   void appendByteStringToken(int kind, ByteString value) {
     // assert(kind != $a || keywords.get(value) == null);
     tail.next = new ByteStringToken(kind, value, tokenStart);
     tail = tail.next;
   }
 
   // This method should be equivalent to the one in super. However,
   // this is a *HOT* method and Dart VM performs better if it is easy
   // to inline.
   int advance() => bytes[++byteOffset];
 }