PercentProperty constructor
Create a diagnostics property for doubles that represent percentages or fractions.
Setting showName to false is often reasonable for PercentProperty
objects, as the fact that the property is shown as a percentage tends to
be sufficient to disambiguate its meaning.
Implementation
PercentProperty(
  super.name,
  super.fraction, {
  super.ifNull,
  super.showName,
  super.tooltip,
  super.unit,
  super.level,
});