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

pkg/http/test/utils.dart

Index: pkg/http/test/utils.dart
diff --git a/pkg/http/test/utils.dart b/pkg/http/test/utils.dart
index 9c6e584d4aedb6468d815b392c9be6e0503da657..800b93a88b494308b057f6819d10ea1458bd43c6 100644
--- a/pkg/http/test/utils.dart
+++ b/pkg/http/test/utils.dart
@@ -5,6 +5,7 @@
 library test_utils;
 
 import 'dart:async';
+import 'dart:convert';
 import 'dart:io';
 import 'dart:json' as json;
 
@@ -62,7 +63,7 @@ Future startServer() {
         if (encodingName != null) {
           outputEncoding = requiredEncodingForCharset(encodingName);
         } else {
-          outputEncoding = Encoding.ASCII;
+          outputEncoding = ASCII;
         }
 
         response.headers.contentType =