Add IOSink:flush().

sdk/lib/io/http_impl.dart

Index: sdk/lib/io/http_impl.dart
diff --git a/sdk/lib/io/http_impl.dart b/sdk/lib/io/http_impl.dart
index c1ec1bf848d2547a44a5c794844bfd37791bd9f1..2fd3659d374f54aed0f2e016d1f49f5bbf9a8ddb 100644
--- a/sdk/lib/io/http_impl.dart
+++ b/sdk/lib/io/http_impl.dart
@@ -468,6 +468,10 @@ abstract class _HttpOutboundMessage<T> implements IOSink {
     return _dataSink.addStream(stream);
   }
 
+  Future flush() {
+    return _dataSink.flush();
+  }
+
   Future close() {
     return _dataSink.close();
   }
@@ -2254,6 +2258,8 @@ class _DetachedSocket extends Stream<List<int>> implements Socket {
 
   void destroy() => _socket.destroy();
 
+  Future flush() => _socket.flush();
+
   Future close() => _socket.close();
 
   Future<Socket> get done => _socket.done;