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

pkg/http/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 test_utils;
 
 import 'dart:async';
+import 'dart:convert';
 import 'dart:io';
 import 'dart:json' as json;
 
 import 'package:http/src/byte_stream.dart';
 import 'package:http/src/utils.dart';
 import 'package:unittest/unittest.dart';
 
 import 'safe_http_server.dart';
 
 /// The current server instance.
@@ skipping 37 matching lines @@
         response.close();
         return;
       }
 
       new ByteStream(request).toBytes().then((requestBodyBytes) {
         var outputEncoding;
         var encodingName = request.uri.queryParameters['response-encoding'];
         if (encodingName != null) {
           outputEncoding = requiredEncodingForCharset(encodingName);
         } else {
-          outputEncoding = Encoding.ASCII;
+          outputEncoding = ASCII;
         }
 
         response.headers.contentType =
             new ContentType(
                 "application", "json", charset: outputEncoding.name);
         response.headers.set('single', 'value');
 
         var requestBody;
         if (requestBodyBytes.isEmpty) {
           requestBody = null;
         } else if (request.headers.contentType.charset != null) {
           var encoding = requiredEncodingForCharset(
               request.headers.contentType.charset);
-          requestBody = decodeString(requestBodyBytes, encoding);
+          requestBody = encoding.decode(requestBodyBytes);
         } else {
           requestBody = requestBodyBytes;
         }
 
         var content = {
           'method': request.method,
           'path': request.uri.path,
           'headers': {}
         };
         if (requestBody != null) content['body'] = requestBody;
@@ skipping 109 matching lines @@
 const isSocketException = const _SocketException();
 
 /// A matcher for functions that throw SocketException.
 const Matcher throwsSocketException =
     const Throws(isSocketException);
 
 class _SocketException extends TypeMatcher {
   const _SocketException() : super("SocketException");
   bool matches(item, Map matchState) => item is SocketException;
 }