Remove usage of dart:utf from dart:io.

tests/standalone/io/http_proxy_test.dart

 // Copyright (c) 2013, 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 "package:crypto/crypto.dart";
 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;
   bool secure;
   int proxyHops;
   List<String> directRequestPaths;
   int requestCount = 0;
 
   Server(this.proxyHops, this.directRequestPaths, this.secure);
 
@@ skipping 135 matching lines @@
             return;
           } else {
             Expect.equals(
                 1, request.headers[HttpHeaders.PROXY_AUTHORIZATION].length);
             String authorization =
               request.headers[HttpHeaders.PROXY_AUTHORIZATION][0];
             if (authScheme == "Basic") {
               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;
               }
             } else {
               HeaderValue header =
                   HeaderValue.parse(
                       authorization, parameterSeparator: ",");
               Expect.equals("Digest", header.value);
               var uri = header.parameters["uri"];
@@ skipping 583 matching lines @@
   testProxyFromEnviroment();
   // The two invocations of uses the same global variable for state -
   // run one after the other.
   testProxyAuthenticate(false)
       .then((_) => testProxyAuthenticate(true));
   // This test is not normally run. It can be used for locally testing
   // with a real proxy server (e.g. Apache).
   //testRealProxy();
   //testRealProxyAuth();
 }