Try to retain original structure of URI.

tests/corelib/uri_test.dart

 // Copyright (c) 2012, 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.
 
 library uriTest;
 
 import "package:expect/expect.dart";
 import 'dart:convert';
 
 testUri(String uri, bool isAbsolute) {
@@ skipping 334 matching lines @@
 
   uri = new Uri(scheme: "file", path: "//y");
   Expect.equals("//y", uri.path);
   Expect.equals("file:////y", uri.toString());
 
   // File scheme noralizes to always showing authority, even if empty.
   uri = new Uri(scheme: "file", path: "/y");
   Expect.equals("file:///y", uri.toString());
   uri = new Uri(scheme: "file", path: "y");
   Expect.equals("file:///y", uri.toString());
+
+  // Empty host/query/fragment ensures the delimiter is there.
+  // Different from not being there.
+  Expect.equals("scheme:/", Uri.parse("scheme:/").toString());
+  Expect.equals("scheme:/",
+                new Uri(scheme: "scheme", path: "/").toString());
+
+  Expect.equals("scheme:///?#", Uri.parse("scheme:///?#").toString());
+  Expect.equals("scheme:///?#",
+                new Uri(scheme: "scheme", host: "", path: "/",
+                        query: "", fragment: "").toString());
 }
 
 main() {
   testUri("http:", true);
   testUri("file://", true);
   testUri("file", false);
   testUri("http://user@example.com:8080/fisk?query=89&hest=silas", true);
   testUri("http://user@example.com:8080/fisk?query=89&hest=silas#fragment",
           false);
   Expect.stringEquals("http://user@example.com/a/b/c?query#fragment",
                       new Uri(
                           scheme: "http",
                           userInfo: "user",
                           host: "example.com",
                           port: 80,
                           path: "/a/b/c",
                           query: "query",
                           fragment: "fragment").toString());
-  Expect.stringEquals("//null@null/a/b/c/",
+  Expect.stringEquals("/a/b/c/",
                       new Uri(
                           scheme: null,
                           userInfo: null,
                           host: null,
                           port: 0,
                           path: "/a/b/c/",
                           query: null,
                           fragment: null).toString());
   Expect.stringEquals("file://", Uri.parse("file:").toString());
 
@@ skipping 139 matching lines @@
 String dump(Uri uri) {
   return "URI: $uri\n"
       "  Scheme:    ${uri.scheme} #${uri.scheme.length}\n"
       "  User-info: ${uri.userInfo} #${uri.userInfo.length}\n"
       "  Host:      ${uri.host} #${uri.host.length}\n"
       "  Port:      ${uri.port}\n"
       "  Path:      ${uri.path} #${uri.path.length}\n"
       "  Query:     ${uri.query} #${uri.query.length}\n"
       "  Fragment:  ${uri.fragment} #${uri.fragment.length}\n";
 }