Remove self-package import (#8483)

This is apperently breaking some customers.

If this gets submitted, we should investigate why self-package imports
don't work for certain customers.

examples/flutter_gallery/lib/demo/all.dart

@@ -33,8 +33,6 @@ export 'snack_bar_demo.dart';
 export 'tabs_demo.dart';
 export 'tabs_fab_demo.dart';
 export 'text_field_demo.dart';
+export 'tooltip_demo.dart';
 export 'two_level_list_demo.dart';
 export 'typography_demo.dart';
-
-// Intentionally use a self-package URL to make sure we don't break these.
-export 'package:flutter_gallery/demo/tooltip_demo.dart';