Is your feature request related to a problem? Please describe.
Gallery uses Flare to display the animated open/close icon on the backdrop on the home page.
There are no plans to migrate Flare to NNBD yet, so we will rewrite the animated icon without using Flare, so that Gallery can be migrated successfully.

Describe the solution you'd like

1. Re-create the animated open/close icon using CustomPaint or a similar widget.
2. Create one or more golden tests.

Describe alternatives you've considered
(No alternatives yet.)

Additional context
(No additional context yet.)