Debug printing widgets was quite verbose. While developing a widget, you
often want to see a full debug printout, but this feature assumes that
debug printing a WidgetInstance should show a summary of the widget, not
a full debug printout containing cached glyph information of every
label.
By default, summarize just calls Debug, but this extra layer allows
widgets to provide a more condensed summary and exclude details like
caches.
Originally, adding dbg!() around the theme example's UI yielded a
whopping 20,324 lines of text. The summary code only prints 3,858
lines.