Add property base to the Uri class

tests/standalone/io/uri_platform_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.
 
 import "package:expect/expect.dart";
 import "dart:io";
 
 main() {
   if (Platform.isWindows) {
     Expect.equals("a\\b", Uri.parse("a/b").toFilePath());
@@ skipping 18 matching lines @@
     Expect.equals("/a b", Uri.parse("file:///a%20b").toFilePath());
     Expect.equals("/a/b", Uri.parse("file:///a/b").toFilePath());
     Expect.equals("/C:", Uri.parse("file:///C:").toFilePath());
     Expect.equals("/C:/", Uri.parse("file:///C:/").toFilePath());
     Expect.throws(() => Uri.parse("file://host/a/b").toFilePath(),
                   (e) => e is UnsupportedError);
 
     Expect.equals("a/b", new Uri.file("a/b").toFilePath());
     Expect.equals("a\\b", new Uri.file("a\\b").toFilePath());
   }
+
+  Expect.equals(Uri.base,
+                new Uri.file(Directory.current.path + Platform.pathSeparator));
 }