Polyfill: Change check to assertion in LeapYear

[catamorphism]

As far as I can see, this function shouldn't be called on undefined.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.76%. Comparing base (03f9057) to head (85c877c).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3232   +/-   ##
=======================================
  Coverage   96.75%   96.76%           
=======================================
  Files          22       22           
  Lines       10398    10398           
  Branches     1859     1858    -1     
=======================================
+ Hits        10061    10062    +1     
+ Misses        289      288    -1     
  Partials       48       48           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.