Text handled by a specialized render object. The vast majority
of render objects are ignorant of the complexities of text. Instead,
text is handled by a specialized render object, RenderParagraph,
which is a leaf in the render tree. Rather than subclassing a
text-aware render object, developers incorporate text into their
user interface using composition. This pattern means RenderParagraph
can avoid recomputing its text layout as long as its parent supplies
the same layout constraints, which is common, even during tree surgery.