Implement least upper bound.

sdk/lib/io/http_impl.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of dart.io;
 
 class _HttpIncoming extends Stream<List<int>> {
   final int _transferLength;
   final Completer _dataCompleter = new Completer();
   Stream<List<int>> _stream;
@@ skipping 1701 matching lines @@
         }
         connection.stopTimer();
         _activeConnections.add(connection);
         _updateTimers();
         return new Future.value(new _ConnnectionInfo(connection, proxy));
       }
       var currentBadCertificateCallback = _badCertificateCallback;
       bool callback(X509Certificate certificate) =>
           currentBadCertificateCallback == null ? false :
           currentBadCertificateCallback(certificate, uriHost, uriPort);
-      return (isSecure && proxy.isDirect
-                  ? SecureSocket.connect(host,
-                                         port,
-                                         sendClientCertificate: true,
-                                         onBadCertificate: callback)
-                  : Socket.connect(host, port))
-        .then((socket) {
+      Future socketFuture = (isSecure && proxy.isDirect
+          ? SecureSocket.connect(host,
+                                 port,
+                                 sendClientCertificate: true,
+                                 onBadCertificate: callback)
+          : Socket.connect(host, port));
+      return socketFuture.then((socket) {
           socket.setOption(SocketOption.TCP_NODELAY, true);
           var connection = new _HttpClientConnection(key, socket, this);
           if (isSecure && !proxy.isDirect) {
             connection._dispose = true;
             return connection.createProxyTunnel(
                 uriHost, uriPort, proxy, callback)
                 .then((tunnel) {
                   _activeConnections.add(tunnel);
                   return new _ConnnectionInfo(tunnel, proxy);
                 });
@@ skipping 782 matching lines @@
   final Uri location;
 }
 
 String _getHttpVersion() {
   var version = Platform.version;
   // Only include major and minor version numbers.
   int index = version.indexOf('.', version.indexOf('.') + 1);
   version = version.substring(0, index);
   return 'Dart/$version (dart:io)';
 }