There are at least three equivalence classes where this assertion
should not actually hold:

  '\u0390\u1fd3',              // ΐΐ
  '\u03b0\u1fe3',              // ΰΰ
  '\ufb05\ufb06',              // ſtst

Bug: v8:10591
Change-Id: I26cb43d2e67c54e689f1831ea13be46c73d5e92d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2231595Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68246}