Add fromUri constructor to File, Directory and Link

sdk/lib/io/file_impl.dart

Index: sdk/lib/io/file_impl.dart
diff --git a/sdk/lib/io/file_impl.dart b/sdk/lib/io/file_impl.dart
index e7080d73ff14ef8242361da56f3f5f43fedc3ce7..00559538447f02c089d9ed863758ddb0cc360821 100644
--- a/sdk/lib/io/file_impl.dart
+++ b/sdk/lib/io/file_impl.dart
@@ -233,6 +233,9 @@ class _File extends FileSystemEntity implements File {
     }
   }
 
+  // Constructor for file from a URI.
+  factory _File.fromUri(Uri uri) => new _File(uri.toFilePath());
+
   Future<bool> exists() {
     return _IOService.dispatch(_FILE_EXISTS, [path]).then((response) {
       if (_isErrorResponse(response)) {