Unregister callbacks from socket, when we return the socket for reuse.

runtime/bin/http_impl.dart

Index: runtime/bin/http_impl.dart
diff --git a/runtime/bin/http_impl.dart b/runtime/bin/http_impl.dart
index 05b1a38b8f105e79425e52d2d345704c36ab6d06..ae9243af27be45418a69f3e261deb8e04e488f22 100644
--- a/runtime/bin/http_impl.dart
+++ b/runtime/bin/http_impl.dart
@@ -1347,6 +1347,9 @@ class _HttpClient implements HttpClient {
   }
 
   void _returnSocketConnection(_SocketConnection socketConn) {
+    // Mark socket as returned to unregister from the old connection.
+    socketConn._markReturned();
+
     // If the HTTP client is beeing shutdown don't return the connection.
     if (_shutdown) {
       socketConn._socket.close();
@@ -1388,7 +1391,6 @@ class _HttpClient implements HttpClient {
     // Return connection.
     _activeSockets.remove(socketConn);
     sockets.addFirst(socketConn);
-    socketConn._markReturned();
   }
 
   Function _onOpen;