Add property base to the Uri class

runtime/lib/uri_patch.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.
 
 // VM implementation of Uri.
+typedef Uri _UriBaseClosure();
+
+void _unsupportedUriBase() {
+  throw new UnsupportedError("'Uri.base' is not supported");
+}
+
+// _uriBaseClosure can be overwritten by the embedder to supply a different
+// value for Uri.base.
+_UriBaseClosure _uriBaseClosure = _unsupportedUriBase;
+
 patch class Uri {
   static final bool _isWindowsCached = _isWindowsPlatform;
 
+  /* patch */ static bool get _isWindows => _isWindowsCached;
+
+  /* patch */ static Uri get base => _uriBaseClosure();
+
   static bool get _isWindowsPlatform native "Uri_isWindowsPlatform";
-
-  /* patch */ static bool get _isWindows => _isWindowsCached;
 }