Make Shrine shopping cart icons display correctly

[pennzht]

Fixes the issue where shopping cart icons cannot be displayed.

Fixes #650

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read the [Flutter Style Guide] recently, and have followed its advice.
• I signed the [CLA].
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
  • This is a bug fix; no documentation needed.
• I added new tests to check the change I am making, or this PR is [test-exempt].
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on [Discord].

[guidezpl]

  _    ___ _____ __  __ 
 | |  / __|_   _|  \/  |
 | |_| (_ | | | | |\/| |
 |____\___| |_| |_|  |_|
                        

Nice!