Remove usage of dart:utf from dart:io.

tests/standalone/io/http_proxy_test.dart

Index: tests/standalone/io/http_proxy_test.dart
diff --git a/tests/standalone/io/http_proxy_test.dart b/tests/standalone/io/http_proxy_test.dart
index 3b56004e08fe39d606526f135c39a35bf3b24189..81806ab65c2c49b88ab17ba2bf85198b1979d41e 100644
--- a/tests/standalone/io/http_proxy_test.dart
+++ b/tests/standalone/io/http_proxy_test.dart
@@ -7,7 +7,7 @@ import "package:expect/expect.dart";
 import "package:path/path.dart";
 import "dart:async";
 import "dart:io";
-import 'dart:utf';
+import 'dart:convert';
 
 class Server {
   HttpServer server;
@@ -163,7 +163,7 @@ class ProxyServer {
               List<String> tokens = authorization.split(" ");
               Expect.equals("Basic", tokens[0]);
               String auth =
-                  CryptoUtils.bytesToBase64(encodeUtf8("$username:$password"));
+                  CryptoUtils.bytesToBase64(UTF8.encode("$username:$password"));
               if (auth != tokens[1]) {
                 basicAuthenticationRequired(request);
                 return;