Add fromUri constructor to File, Directory and Link

sdk/lib/io/directory_impl.dart

Index: sdk/lib/io/directory_impl.dart
diff --git a/sdk/lib/io/directory_impl.dart b/sdk/lib/io/directory_impl.dart
index 9cbd1b9661d198a52976c78b435e1b8df6e60e04..939be7b85c600f2ab62211b13e608c72aa36888c 100644
--- a/sdk/lib/io/directory_impl.dart
+++ b/sdk/lib/io/directory_impl.dart
@@ -14,6 +14,8 @@ class _Directory extends FileSystemEntity implements Directory {
     }
   }
 
+  factory _Directory.fromUri(Uri uri) => new _Directory(uri.toFilePath());
+
   external static _current();
   external static _setCurrent(path);
   external static _createTemp(String path);