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

pkg/http_server/test/utils.dart

Index: pkg/http_server/test/utils.dart
diff --git a/pkg/http_server/test/utils.dart b/pkg/http_server/test/utils.dart
index 6a46696cff4cbee8dc586181660fed24dc046889..c9e39c7c61bdefb0f2001cea742e6366c2ac0984 100644
--- a/pkg/http_server/test/utils.dart
+++ b/pkg/http_server/test/utils.dart
@@ -5,6 +5,7 @@
 library utils;
 
 import 'dart:async';
+import 'dart:convert';
 import 'dart:io';
 import "package:path/path.dart";
 
@@ -49,7 +50,7 @@ Future<HttpHeaders> getHeaders(int port, String path) {
 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));
 }