Add property base to the Uri class

runtime/lib/uri_patch.dart

Index: runtime/lib/uri_patch.dart
diff --git a/runtime/lib/uri_patch.dart b/runtime/lib/uri_patch.dart
index b07946d190d276da4dabe4f85cd104a1e05475a5..1f94bd7038169c9ce2fa8dee8e83499fdc214d08 100644
--- a/runtime/lib/uri_patch.dart
+++ b/runtime/lib/uri_patch.dart
@@ -3,10 +3,22 @@
 // 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;
 
-  static bool get _isWindowsPlatform native "Uri_isWindowsPlatform";
-
   /* patch */ static bool get _isWindows => _isWindowsCached;
+
+  /* patch */ static Uri get base => _uriBaseClosure();
+
+  static bool get _isWindowsPlatform native "Uri_isWindowsPlatform";
 }