Fix all strong-mode warnings.

lib/src/utils.dart

 // Copyright (c) 2012, 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 'dart:convert';
 
-import 'package:crypto/crypto.dart';
-
 /// Adds additional query parameters to [url], overwriting the original
 /// parameters if a name conflict occurs.
 Uri addQueryParameters(Uri url, Map<String, String> parameters) => url.replace(
     queryParameters: new Map.from(url.queryParameters)..addAll(parameters));
 
 String basicAuthHeader(String identifier, String secret) {
   var userPass = Uri.encodeFull(identifier) + ":" + Uri.encodeFull(secret);
-  return "Basic " + CryptoUtils.bytesToBase64(ASCII.encode(userPass));
+  return "Basic " + BASE64.encode(ASCII.encode(userPass));
 }