macOS was sharing the iOS font theme, but the names of the underlying
fonts differ between the two platforms, so this was actually using the
fallback font from the engine. Instead of using the underlying font
names on macOS, which is no longer allowed by CoreText, use the
meta-font returned by systemFontOfSize:

This has been verified on 10.15; older versions of macOS may need
different font names as the handling of these fonts have changed over
time. Fixing older versions as necessary will be handled in a follow-up.

See https://github.com/flutter/flutter/issues/57805