Update deprecations.

lib/src/digest.dart

 // Copyright (c) 2015, 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.
 
-import 'crypto_utils.dart';
+import 'package:convert/convert.dart';
 
 /// A message digest as computed by a [Hash] or [HMAC] function.
 class Digest {
   /// The message digest as an array of bytes.
   final List<int> bytes;
 
   Digest(this.bytes);
 
   /// Returns whether this is equal to another digest.
   ///
   /// This should be used instead of manual comparisons to avoid leaking
   /// information via timing.
   bool operator ==(Object other) {
     if (other is! Digest) return false;
 
     var digest = other as Digest;
     if (digest.bytes.length != bytes.length) return false;
 
     var result = 0;
     for (var i = 0; i < bytes.length; i++) {
       result |= bytes[i] ^ digest.bytes[i];
     }
     return result == 0;
   }
 
   /// The message digest as a string of hexadecimal digits.
-  String toString() => CryptoUtils.bytesToHex(bytes);
+  String toString() => hex.encode(bytes);
 }