Remove Encoding-enum from dart:io and add interface in dart:convert.

pkg/http_server/test/utils.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.
 
 library utils;
 
 import 'dart:async';
+import 'dart:convert';
 import 'dart:io';
 import "package:path/path.dart";
 
 Future<int> getStatusCode(int port,
                           String path,
                           {String host,
                            bool secure: false,
                            DateTime ifModifiedSince,
                            bool rawPath: false}) {
   var uri;
@@ skipping 24 matching lines @@
   return new HttpClient().get('localhost', port, path)
       .then((request) => request.close())
       .then((response) => response.drain().then(
           (_) => response.headers));
 }
 
 
 Future<String> getAsString(int port, String path) {
   return new HttpClient().get('localhost', port, path)
       .then((request) => request.close())
-      .then((response) => StringDecoder.decode(response));
+      .then((response) => UTF8.decodeStream(response));
 }
 
 
 const CERTIFICATE = "localhost_cert";
 
 
 setupSecure() {
   String scriptDir = dirname(new Options().script);
   String certificateDatabase = join(scriptDir, 'pkcert');
   SecureSocket.initialize(database: certificateDatabase,
                           password: 'dartdart');
 }