Add property base to the Uri class

sdk/lib/core/uri.dart

Index: sdk/lib/core/uri.dart
diff --git a/sdk/lib/core/uri.dart b/sdk/lib/core/uri.dart
index 328b9950d047151eba55b8bc59bb43463d325856..b2aaa42a085c5997b80c5650eadbb92b879ffb0e 100644
--- a/sdk/lib/core/uri.dart
+++ b/sdk/lib/core/uri.dart
@@ -358,6 +358,17 @@ class Uri {
     return windows ? _makeWindowsFileUrl(path) : _makeFileUri(path);
   }
 
+  /**
+   * Returns the natural base URI for the current platform.
+   *
+   * When running in the browser this will be the current URL (from
+   * `window.location.href`).
+   *
+   * When running in the standalone VM this will be a file URI
+   * referencing the current working directory.
+   */
+  external static Uri get base;
+
   external static bool get _isWindows;
 
   static _checkNonWindowsPathReservedCharacters(List<String> segments,