Creates a text painter that paints the given text.
The text argument is optional but text must be non-null before calling
layout.
Source
TextPainter({
TextSpan text,
TextAlign textAlign,
double textScaleFactor: 1.0,
String ellipsis,
}) : _text = text, _textAlign = textAlign, _textScaleFactor = textScaleFactor, _ellipsis = ellipsis {
assert(text == null || text.debugAssertIsValid());
assert(textScaleFactor != null);
}