Remove usage of dart:utf from dart:io.

tests/standalone/io/http_auth_test.dart

Index: tests/standalone/io/http_auth_test.dart
diff --git a/tests/standalone/io/http_auth_test.dart b/tests/standalone/io/http_auth_test.dart
index 9869a94bcec2b058e328bf3eba3994f3ddbd37db..d259bdf030001b1feda5deb66d2d332a4c900ef5 100644
--- a/tests/standalone/io/http_auth_test.dart
+++ b/tests/standalone/io/http_auth_test.dart
@@ -7,7 +7,7 @@ import "package:expect/expect.dart";
 import 'dart:async';
 import 'dart:io';
 import 'dart:isolate';
-import 'dart:utf';
+import 'dart:convert';
 
 class Server {
   HttpServer server;
@@ -42,7 +42,7 @@ class Server {
           List<String> tokens = authorization.split(" ");
           Expect.equals("Basic", tokens[0]);
           String auth =
-              CryptoUtils.bytesToBase64(encodeUtf8("$username:$password"));
+              CryptoUtils.bytesToBase64(UTF8.encode("$username:$password"));
           if (passwordChanged && auth != tokens[1]) {
             response.statusCode = HttpStatus.UNAUTHORIZED;
             response.headers.set(HttpHeaders.WWW_AUTHENTICATE,