Fix all strong-mode warnings.

lib/src/utils.dart

Index: lib/src/utils.dart
diff --git a/lib/src/utils.dart b/lib/src/utils.dart
index e7cc550807ab18b1dc85f6743b631f9ddd231ef2..150fa174a57573de7223af9e312849e760288445 100644
--- a/lib/src/utils.dart
+++ b/lib/src/utils.dart
@@ -4,8 +4,6 @@
 
 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(
@@ -13,5 +11,5 @@ Uri addQueryParameters(Uri url, Map<String, String> parameters) => url.replace(
 
 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));
 }