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

pkg/http/lib/src/request.dart

Index: pkg/http/lib/src/request.dart
diff --git a/pkg/http/lib/src/request.dart b/pkg/http/lib/src/request.dart
index 9f024e6e95fb78fcf3efeb912b36d70551f5b03c..aa5c5fea959b79fd8caad35c84df06f9d4341f25 100644
--- a/pkg/http/lib/src/request.dart
+++ b/pkg/http/lib/src/request.dart
@@ -4,6 +4,7 @@
 
 library request;
 
+import 'dart:convert';
 import 'dart:io';
 import 'dart:typed_data';
 
@@ -36,7 +37,7 @@ class Request extends BaseRequest {
   /// If the request has a `Content-Type` header and that header has a `charset`
   /// parameter, that parameter's value is used as the encoding. Otherwise, if
   /// [encoding] has been set manually, that encoding is used. If that hasn't
-  /// been set either, this defaults to [Encoding.UTF_8].
+  /// been set either, this defaults to [UTF8].
   ///
   /// If the `charset` parameter's value is not a known [Encoding], reading this
   /// will throw a [FormatException].
@@ -137,7 +138,7 @@ class Request extends BaseRequest {
   /// Creates a new HTTP request.
   Request(String method, Uri url)
     : super(method, url),
-      _defaultEncoding = Encoding.UTF_8,
+      _defaultEncoding = UTF8,
       _bodyBytes = new Uint8List(0);
 
   /// Freezes all mutable fields and returns a single-subscription [ByteStream]