Support redirect to relative Uri, as specified by rfc3986#4.2.

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 1394 matching lines @@
           return info.connection.send(uri,
                                       port,
                                       method.toUpperCase(),
                                       info.proxy);
         });
   }
 
   Future<HttpClientRequest> _openUrlFromRequest(String method,
                                                 Uri uri,
                                                 _HttpClientRequest previous) {
+    // 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, domain, port, path}) {
+      uri = new Uri.fromComponents(
+          scheme: scheme != null ? scheme : uri.scheme,
+          domain: domain != null ? domain : uri.domain,
+          port: port != null ? port : uri.port,
+          path: path != null ? path : uri.path,
+          query: uri.query,
+          fragment: uri.fragment);
+    }
+    if (uri.domain == '') {
+      replaceComponents(domain: previous.uri.domain, port: previous.uri.port);
+    }
+    if (uri.scheme == '') {
+      replaceComponents(scheme: previous.uri.scheme);
+    }
+    if (!uri.path.startsWith('/') && previous.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());
+    }
     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 753 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;
 }