Fix assert.

pkg/barback/lib/src/utils.dart

 // Copyright (c) 2013, 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.
 
 library barback.utils;
 
 /// Converts a number in the range [0-255] to a two digit hex string.
 ///
 /// For example, given `255`, returns `ff`.
 String byteToHex(int byte) {
-  assert(byte >= 0 && byte < 255);
+  assert(byte >= 0 && byte <= 255);
 
   const DIGITS = "0123456789abcdef";
   return DIGITS[(byte ~/ 16) % 16] + DIGITS[byte % 16];
 }