Some more removals of dart:utf.

pkg/http/lib/src/multipart_request.dart

Index: pkg/http/lib/src/multipart_request.dart
diff --git a/pkg/http/lib/src/multipart_request.dart b/pkg/http/lib/src/multipart_request.dart
index e0c413a66db678560c30e1232eea3142aabb3b3f..5bbd5b328497539fa68c4f27e754baf0ed715715 100644
--- a/pkg/http/lib/src/multipart_request.dart
+++ b/pkg/http/lib/src/multipart_request.dart
@@ -5,8 +5,8 @@
 library multipart_request;
 
 import 'dart:async';
+import 'dart:convert';
 import 'dart:math';
-import 'dart:utf';
 
 import 'base_request.dart';
 import 'byte_stream.dart';
@@ -62,7 +62,7 @@ class MultipartRequest extends BaseRequest {
     fields.forEach((name, value) {
       length += "--".length + _BOUNDARY_LENGTH + "\r\n".length +
           _headerForField(name, value).length +
-          encodeUtf8(value).length + "\r\n".length;
+          UTF8.encode(value).length + "\r\n".length;
     });
 
     for (var file in _files) {
@@ -95,7 +95,7 @@ class MultipartRequest extends BaseRequest {
       controller.add(string.codeUnits);
     }
 
-    writeUtf8(String string) => controller.add(encodeUtf8(string));
+    writeUtf8(String string) => controller.add(UTF8.encode(string));
     writeLine() => controller.add([13, 10]); // \r\n
 
     fields.forEach((name, value) {