systemFonts property Null safety
Listenable that notifies when the available fonts on the system have changed.
System fonts can change when the system installs or removes new font. To correctly reflect the change, it is important to relayout text related widgets when this happens.
Objects that show text and/or measure text (e.g. via TextPainter or Paragraph) should listen to this and redraw/remeasure.
Listenable get systemFonts => _systemFonts;