Add writeHeaders argument to HttpResponse::detachSocket.

sdk/lib/io/http.dart

Index: sdk/lib/io/http.dart
diff --git a/sdk/lib/io/http.dart b/sdk/lib/io/http.dart
index b869f39aaef03a8b268026f0d2e5ce801dd0b43d..18360c64d6d85e50a2fb5f5f4dbeefd54a2c4de3 100644
--- a/sdk/lib/io/http.dart
+++ b/sdk/lib/io/http.dart
@@ -1128,8 +1128,12 @@ abstract class HttpResponse implements IOSink {
    *
    * This is normally used when a HTTP upgrade request is received
    * and the communication should continue with a different protocol.
+   *
+   * If [writeHeaders] is `true`, the status line and [headers] will be written
+   * to the socket before it's detached. If `false`, the socket is detached
+   * immediately, without any data written to the socket. Default is `true`.
    */
-  Future<Socket> detachSocket();
+  Future<Socket> detachSocket({bool writeHeaders: true});
 
   /**
    * Gets information about the client connection. Returns [:null:] if the