Add leading slash to URI path when required.

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 1467 matching lines @@
             return _getConnection(uri.host, port, proxyConf, isSecure)
                 .then(send);
           }
           return send(info);
         });
   }
 
   Future<HttpClientRequest> _openUrlFromRequest(String method,
                                                 Uri uri,
                                                 _HttpClientRequest previous) {
+    var u = uri;
     // If the new URI is relative (to either '/' or some sub-path),
     // construct a full URI from the previous one.
     // See http://tools.ietf.org/html/rfc3986#section-4.2
     replaceComponents({scheme, host, port, path}) {
       uri = new Uri(
           scheme: scheme != null ? scheme : uri.scheme,
           host: host != null ? host : uri.host,
           port: port != null ? port : uri.port,
           path: path != null ? path : uri.path,
           query: uri.query,
           fragment: uri.fragment);
     }
+
+    var scheme;
+    var host;
+    var port;
+    var path;
     if (uri.host.isEmpty) {
-      replaceComponents(host: previous.uri.host, port: previous.uri.port);
+      host = previous.uri.host;
+      port = previous.uri.port;
     }
     if (uri.scheme.isEmpty) {
-      replaceComponents(scheme: previous.uri.scheme);
+      scheme = previous.uri.scheme;
     }
-    if (!uri.path.startsWith('/') && previous.uri.path.startsWith('/')) {
+    if (!uri.path.startsWith('/')) {
       var absolute = new Path.raw(previous.uri.path).directoryPath;
-      absolute = absolute.join(new Path.raw(uri.path));
-      replaceComponents(path: absolute.canonicalize().toString());
+      absolute = absolute.join(new Path.raw(u.path));
+      path = absolute.canonicalize().toString();
     }
+    replaceComponents(scheme: scheme, host: host, port: port, path: path);
     return openUrl(method, uri).then((_HttpClientRequest request) {
           // Only follow redirects if initial request did.
           request.followRedirects = previous.followRedirects;
           // Allow same number of redirects.
           request.maxRedirects = previous.maxRedirects;
           // Copy headers.
           for (var header in previous.headers._headers.keys) {
             if (request.headers[header] == null) {
               request.headers.set(header, previous.headers[header]);
             }
@@ skipping 808 matching lines @@
 
 
 class _RedirectInfo implements RedirectInfo {
   const _RedirectInfo(int this.statusCode,
                       String this.method,
                       Uri this.location);
   final int statusCode;
   final String method;
   final Uri location;
 }